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Intro

This is a Docker introduction -- this assumes Docker is already installed in your machine

Ops​

docker version​

command to test that the client and daemon (server) are running and talking to each other

bash

// this shows client is talking to the server
$ docker version

Client:
 Version:           24.0.5
 API version:       1.43
 Go version:        go1.22.0
 Git commit:        %{shortcommit_cli}
 Built:             Sun Feb 11 23:35:47 2024
 OS/Arch:           linux/amd64
 Context:           default

Server:
 Engine:
  Version:          24.0.5
  API version:      1.43 (minimum version 1.12)
  Go version:       go1.22.0
  Git commit:       %{shortcommit_moby}
  Built:            Sun Feb 11 23:35:47 2024
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          1.6.23
  GitCommit:
 runc:
  Version:          1.3.0
  GitCommit:
 docker-init:
  Version:          0.19.0
  GitCommit:

// stopping docker - shows server not running
$ sudo systemctl stop docker.service docker.socket containerd

@:~$ docker version

Client:
 Version:           24.0.5
 API version:       1.43
 Go version:        go1.22.0
 Git commit:        %{shortcommit_cli}
 Built:             Sun Feb 11 23:35:47 2024
 OS/Arch:           linux/amd64
 Context:           default
Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?

docker images​

Docker image is like an object that contains an OS filesystem, an application, and all application dependencies.
An image gets its own unique ID.
When referencing images, you can refer to them using either IDs or names.

bash
// currently no images
$ docker images
REPOSITORY   TAG       IMAGE ID   CREATED   SIZE

// Getting images onto your Docker host is called pulling
// sample pull of ubuntu image
$ docker pull ubuntu:latest
latest: Pulling from library/ubuntu
20043066d3d5: Pull complete
Digest: sha256:c35e29c9450151419d9448b0fd75374fec4fff364a27f176fb458d472dfc9e54
Status: Downloaded newer image for ubuntu:latest
docker.io/library/ubuntu:latest

// docker after pull - showing ubuntu image
$ docker images
REPOSITORY   TAG       IMAGE ID       CREATED        SIZE
ubuntu       latest    c3a134f2ace4   2 months ago   78.1MB

docker containers​

docker run command to launch a container from the ubuntu image
The -it flags tell Docker
to make the container interactive
and to attach the current shell to the container’s terminal

bash
// issuing some commands inside the running ubuntu image container
$ docker run -it ubuntu:latest /bin/bash
root@a1bb606e08be:/#
root@a1bb606e08be:/#
root@a1bb606e08be:/# hostname
a1bb606e08be
root@a1bb606e08be:/# uname -a
Linux a1bb606e08be 6.14.5-100.fc40.x86_64 #1 SMP PREEMPT_DYNAMIC Fri May  2 14:22:13 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux
root@a1bb606e08be:/# date
Wed Dec 31 13:57:37 UTC 2025
root@a1bb606e08be:/# whoami
root
root@a1bb606e08be:/# ps
    PID TTY          TIME CMD
      1 pts/0    00:00:00 bash
     13 pts/0    00:00:00 ps
root@a1bb606e08be:/#

Exit container without terminating​

Press Ctrl-PQ to exit the container without terminating it.
Exit from the container without killing it.
docker ps command - to see all running containers

bash

// docker ps showing active running containers
// below showing the ubuntu image container running - 26 minutes
$ docker ps
CONTAINER ID   IMAGE           COMMAND       CREATED          STATUS          PORTS     NAMES
a1bb606e08be   ubuntu:latest   "/bin/bash"   26 minutes ago   Up 26 minutes             youthful_vaughan

Connecting to running containers​

bash
// using CONTAINER NAME
$ docker exec -it youthful_vaughan bash
root@a1bb606e08be:/#
root@a1bb606e08be:/# read escape sequence
$

// using CONTAINER ID
$ docker exec -it a1bb606e08be bash
root@a1bb606e08be:/#
root@a1bb606e08be:/#
root@a1bb606e08be:/# read escape sequence
$
$

// using shortcut - first few characters unique part of the CONTAINER ID
// as long as it is unique
$ docker exec -it a1 bash
root@a1bb606e08be:/#
root@a1bb606e08be:/# read escape sequence
$
$

Stopping / Killing running containers​

Stop the container and kill it
using the docker stop CONTAINER_ID | NAME -- to stop a running container
and docker rm CONTAINER_ID | NAME command to remove a container -- only after it has stopped
substitute the names/IDs of the containers.

After container is stopped
Remove Images you want deleted
Use docker rmi REPOSITORY[:TAG] | IMAGE_ID

bash
// check running containers
$ docker ps
CONTAINER ID   IMAGE           COMMAND       CREATED             STATUS             PORTS     NAMES
a1bb606e08be   ubuntu:latest   "/bin/bash"   About an hour ago   Up About an hour             youthful_vaughan
$

// unsuccessful removal of still running container
$ docker rm a1bb606e08be
Error response from daemon: You cannot remove a running container a1bb606e08bedc6a05b8696f08d7790763baa60426d7220359d5115bb860af45. Stop the container before attempting removal or force remove
$
$

// stoping a running container
$ docker stop a1bb606e08be
a1bb606e08be
$

// docker ps  -- show container removed from the list
$ docker ps
CONTAINER ID   IMAGE     COMMAND   CREATED   STATUS    PORTS     NAMES

// docker ps -a -- still showing container
$ docker ps -a
CONTAINER ID   IMAGE           COMMAND       CREATED             STATUS                        PORTS     NAMES
a1bb606e08be   ubuntu:latest   "/bin/bash"   About an hour ago   Exited (137) 22 seconds ago             youthful_vaughan
$

// safe - successful removal of a stopped container
$ docker rm a1bb606e08be
a1bb606e08be
$
$ docker ps -a
CONTAINER ID   IMAGE     COMMAND   CREATED   STATUS    PORTS     NAMES
$ docker ps
CONTAINER ID   IMAGE     COMMAND   CREATED   STATUS    PORTS     NAMES
$
$ docker images
REPOSITORY   TAG       IMAGE ID       CREATED        SIZE
ubuntu       latest    c3a134f2ace4   2 months ago   78.1MB
$
$

// image can be removed only after the container is stopped / removed
$ docker rmi ubuntu
Untagged: ubuntu:latest
Untagged: ubuntu@sha256:c35e29c9450151419d9448b0fd75374fec4fff364a27f176fb458d472dfc9e54
Deleted: sha256:c3a134f2ace4f6d480733efcfef27c60ea8ed48be1cd36f2c17ec0729775b2c8
Deleted: sha256:e8bce0aabd687e9ee90e0bada33884f40b277196f72aac9934357472863a80ae
$


Dev​

clone an app from a Git repo​

bash

$ git clone https://github.com/nigelpoulton/psweb.git
Cloning into 'psweb'...
remote: Enumerating objects: 63, done.
remote: Counting objects: 100% (34/34), done.
remote: Compressing objects: 100% (22/22), done.
remote: Total 63 (delta 13), reused 25 (delta 9), pack-reused 29
Receiving objects: 100% (63/63), 13.29 KiB | 4.43 MiB/s, done.
Resolving deltas: 100% (21/21), done.

inspect its Dockerfile​

bash

$ cd psweb
$ ls -l
total 40
-rw-r--r--@ 1 ubuntu ubuntu
-rw-r--r--@ 1 ubuntu ubuntu
-rw-r--r--@ 1 ubuntu ubuntu
-rw-r--r-- 1 ubuntu ubuntu
-rw-r--r--@ 1 ubuntu ubuntu
drwxr-xr-x 4 ubuntu ubuntu
drwxr-xr-x 3 ubuntu ubuntu
338 24 Apr 19:29 Dockerfile
396 24 Apr 19:32 README.md
341 24 Apr 19:29 app.js
216 24 Apr 19:29 circle.yml
377 24 Apr 19:36 package.json
128 24 Apr 19:29 test
96 24 Apr 19:29 views

$ cat Dockerfile
FROM alpine
LABEL maintainer="nigelpoulton@hotmail.com"
RUN apk add --update nodejs nodejs-npm
COPY . /src
WORKDIR /src
RUN npm install
EXPOSE 8080
ENTRYPOINT ["node", "./app.js"]



containerize and run app container.​


$ docker build -t test:latest .
[+] Building 36.2s (11/11) FINISHED
=> [internal] load .dockerignore
=> => transferring context: 2B
=> [internal] load build definition from Dockerfile
<Snip>
=> => naming to docker.io/library/test:latest
=> => unpacking to docker.io/library/test:latest
0.0s
0.0s
0.0s
0.0s
0.7s

$ docker images
REPO
TAG
test
latest
IMAGE ID
1ede254e072b
CREATED
7 seconds ago
SIZE
154MB

$ docker run -d \
--name web1 \
--publish 8080:8080 \
test:latest

retagging images​

bash
docker images
REPOSITORY                        TAG       IMAGE ID       SIZE
gnostex-backend-api                latest    def456
gnostex-backend-api                abc123    abc123

docker tag abc123 gnostex-backend-api:latest

docker images
REPOSITORY                        TAG       IMAGE ID       SIZE
gnostex-backend-api                latest    abc123
gnostex-backend-api                def456    def456
gnostex-backend-api                abc123    abc123

// Remove old latest tag reference
docker rmi gnostex-backend-api:latest

// Tags are just labels, not copies of images.
// You can move tags freely — this is how you can roll back latest to a previous build.
// Useful for manual rollback on VPS:

docker stop gnostex-backend-api
docker rm gnostex-backend-api
docker tag abc123 gnostex-backend-api:latest
docker run -d --name gnostex-backend-api -p 8080:8080 gnostex-backend-api:latest

Local​

This example shows a basic local setup using Spring Boot, PostgreSQL, and Docker.
The Spring Boot application runs inside a Docker container and connects to a local PostgreSQL database.

set-up​

.env​

[root project directory]/.env
DB_USERNAME=gnostex
DB_PASSWORD=gnostex
DB_URL=jdbc:postgresql://host.docker.internal:5432/gnostexdb
SPRING_PROFILES_ACTIVE=dev

Dockerfile​

[root project directory]/Dockerfile
# =========================
# Build stage
# =========================
FROM maven:3.9.9-eclipse-temurin-21 AS build
WORKDIR /app

COPY pom.xml .
RUN mvn dependency:go-offline

COPY src ./src
RUN mvn -B -Dmaven.test.skip=true clean package



# =========================
# Runtime stage
# =========================
FROM eclipse-temurin:21-jre
WORKDIR /app

# add curl
RUN apt-get update && apt-get install -y curl && rm -rf /var/lib/apt/lists/*

# Copy jar from build stage
COPY --from=build /app/target/gnostex-backend-api-0.0.1-SNAPSHOT.jar app.jar

# Copy source and target for exploration
# Comment out in production
#COPY --from=build /app/src ./src
#COPY --from=build /app/target ./target
#COPY --from=build /app/pom.xml ./pom.xml


# App port
EXPOSE 8080

# Optional JVM tuning (safe defaults)
ENV JAVA_OPTS=""

# health check
HEALTHCHECK --interval=30s --timeout=5s --start-period=20s \
  CMD curl -f http://localhost:8080/actuator/health || exit 1

# Run Spring Boot
ENTRYPOINT ["sh","-c","java $JAVA_OPTS -jar /app/app.jar"]

building​

run the docker command to build the image.
use the -t option to name en image and version label.
point the command where [path] to the Dockerfile location

[root project directory]/ 
// -t tag -- this will set the name of the image / version label
// [.] -- the directory contain the Dockerfile

docker build -t gnostex-api:local .

Use docker images to list all Docker images available on your local machine.
Started with empty list, build, then we have myapp:local listed in the local machine.

sample output
$ docker images
REPOSITORY   TAG       IMAGE ID   CREATED   SIZE


$ docker build -t myapp:local .
Sending build context to Docker daemon  140.8kB
Step 1/14 : FROM maven:3.9.9-eclipse-temurin-21 AS build
3.9.9-eclipse-temurin-21: Pulling from library/maven
d9d352c11bbd: Pull complete
380ab39b4f9f: Pull complete
.
.
.
.
Successfully tagged myapp:local


$ docker images
REPOSITORY        TAG                        IMAGE ID       CREATED              SIZE
myapp             local                      ec2be4ef4307   32 seconds ago       347MB
<none>            <none>                     d1f20d771af3   About a minute ago   777MB
eclipse-temurin   21-jre                     2e463b829a56   7 weeks ago          287MB
maven             3.9.9-eclipse-temurin-21   4efe8769cc32   16 months ago        530MB

running​


$ docker run \
--name myapp-backend \
--env-file .env \
-p 8080:8080 \
--add-host=host.docker.internal:host-gateway  \
myapp:local


  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/

 :: Spring Boot ::                (v3.5.9)

2026-01-07T07:32:56.038Z  INFO 7 --- [springboot-postgresql-docker-githubactions] [           main] PostgresqlDockerGithubactionsApplication : Starting SpringbootPostgresqlDockerGithubactionsApplication v0.0.1-SNAPSHOT using Java 21.0.9 with PID 7 (/app/app.jar started by root in /app)
2026-01-07T07:32:56.041Z  INFO 7 --- [springboot-postgresql-docker-githubactions] [           main] PostgresqlDockerGithubactionsApplication : The following 1 profile is active: "dev"
2026-01-07T07:32:57.028Z  INFO 7 --- [springboot-postgresql-docker-githubactions] [           main] .s.d.r.c.RepositoryConfigurationDelegate : Bootstrapping Spring Data JPA repositories in DEFAULT mode.
2026-01-07T07:32:57.053Z  INFO 7 --- [springboot-postgresql-docker-githubactions] [           main] .s.d.r.c.RepositoryConfigurationDelegate : Finished Spring Data repository scanning in 15 ms. Found 0 JPA reposito
.
.
.
.
2026-01-07T07:33:25.226Z  INFO 7 --- [springboot-postgresql-docker-githubactions] [nio-8080-exec-1] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring DispatcherServlet 'dispatcherServlet'
2026-01-07T07:33:25.227Z  INFO 7 --- [springboot-postgresql-docker-githubactions] [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet        : Initializing Servlet 'dispatcherServlet'
2026-01-07T07:33:25.228Z  INFO 7 --- [springboot-postgresql-docker-githubactions] [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet        : Completed initialization in 0 ms
# from another terminal
$ docker ps
CONTAINER ID   IMAGE         COMMAND                  CREATED              STATUS                        PORTS                                       NAMES
818a93282e59   myapp:local   "sh -c 'java $JAVA_O…"   About a minute ago   Up About a minute (healthy)   0.0.0.0:8080->8080/tcp, :::8080->8080/tcp   myapp-backend

$ docker ps -a
CONTAINER ID   IMAGE         COMMAND                  CREATED         STATUS                   PORTS                                       NAMES
818a93282e59   myapp:local   "sh -c 'java $JAVA_O…"   2 minutes ago   Up 2 minutes (healthy)   0.0.0.0:8080->8080/tcp, :::8080->8080/tcp   myapp-backend

interrogating​

using a browser​

just point your browser to the local URL


http://localhost:8080/actuator/health

using curl​

  • curl with jq -- a commandline JSON processor, used here to prettify JSON
$ curl http://localhost:8080/actuator/health | jq
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   342    0   342    0     0  61301      0 --:--:-- --:--:-- --:--:-- 68400
{
  "status": "UP",
  "components": {
    "db": {
      "status": "UP",
      "details": {
        "database": "PostgreSQL",
        "validationQuery": "isValid()"
      }
    },
    "diskSpace": {
      "status": "UP",
      "details": {
        "total": 603861286912,
        "free": 464497238016,
        "threshold": 10485760,
        "path": "/app/.",
        "exists": true
      }
    },
    "ping": {
      "status": "UP"
    },
    "ssl": {
      "status": "UP",
      "details": {
        "validChains": [],
        "invalidChains": []
      }
    }
  }
}


connecting to the myapp-backend running container​

issuing commands inside the shell.

  • use docker ps to check images that are running
  • once you have identified the container you wanted to connect to
    issue docker -exec -it [cotaniner id]|[name] sh
    -exec used to issued command to a running cotainer
    -it interactive with tty allocated

$ docker ps
CONTAINER ID   IMAGE         COMMAND                  CREATED          STATUS                    PORTS                                       NAMES
818a93282e59   myapp:local   "sh -c 'java $JAVA_O…"   31 minutes ago   Up 30 minutes (healthy)   0.0.0.0:8080->8080/tcp, :::8080->8080/tcp   myapp-backend


$ docker exec -it myapp-backend sh
# hostname
818a93282e59
# uname -a
Linux 818a93282e59 6.14.5-100.fc40.x86_64 #1 SMP PREEMPT_DYNAMIC Fri May  2 14:22:13 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux
# date
Wed Jan  7 07:57:15 AM UTC 2026
# pwd
/app
# ls
app.jar
# ps -ef
UID          PID    PPID  C STIME TTY          TIME CMD
root           1       0  0 07:32 ?        00:00:00 sh -c java $JAVA_OPTS -jar /app/app.jar
root           7       1  1 07:32 ?        00:00:29 java -jar /app/app.jar
root         405       0  0 07:56 pts/0    00:00:00 sh
root         411       0  0 07:56 pts/1    00:00:00 sh
root         417       0  0 07:56 pts/2    00:00:00 sh
root         430       0  0 07:57 pts/3    00:00:00 sh
root         449     430  0 07:57 pts/3    00:00:00 ps -ef
#


cleaning up​

resetting and clearing all containers / images

// checking available images locally
$ docker images
REPOSITORY   TAG       IMAGE ID       CREATED          SIZE
myapp        local     ec2be4ef4307   58 minutes ago   347MB
$

// trying to remove an image that is currently being used by a container
$ docker rmi myapp:local
Error response from daemon: conflict: unable to remove repository reference "myapp:local" (must force) - container 818a93282e59 is using its referenced image ec2be4ef4307
$
$

// using docker ps command check running container
$ docker ps
CONTAINER ID   IMAGE         COMMAND                  CREATED          STATUS                    PORTS                                       NAMES
818a93282e59   myapp:local   "sh -c 'java $JAVA_O…"   47 minutes ago   Up 47 minutes (healthy)   0.0.0.0:8080->8080/tcp, :::8080->8080/tcp   myapp-backend

// docker ps -a give a list of all containers, running or exited (but still being referenced)
$ docker ps -a
CONTAINER ID   IMAGE         COMMAND                  CREATED          STATUS                    PORTS                                       NAMES
818a93282e59   myapp:local   "sh -c 'java $JAVA_O…"   48 minutes ago   Up 48 minutes (healthy)   0.0.0.0:8080->8080/tcp, :::8080->8080/tcp   myapp-backend
$
$

// stopping ` myapp-backend ` container
$ docker stop myapp-backend
myapp-backend

// ` docker ps ` now showing NO RUNNING CONTAINER after ` myapp-backend ` was stopped
$ docker ps
CONTAINER ID   IMAGE     COMMAND   CREATED   STATUS    PORTS     NAMES

// attempt to remove the image ` myapp:local ` - failed since image is still being refrenced
$ docker rmi myapp:local
Error response from daemon: conflict: unable to remove repository reference "myapp:local" (must force) - container 818a93282e59 is using its referenced image ec2be4ef4307
$
$

// ` docker ps -a ` show image ` myapp:local ` though stopped still is being referenced inside the ` myapp-backend `
$ docker ps -a
CONTAINER ID   IMAGE         COMMAND                  CREATED          STATUS                        PORTS     NAMES
818a93282e59   myapp:local   "sh -c 'java $JAVA_O…"   48 minutes ago   Exited (137) 23 seconds ago             myapp-backend
$
$

// removing the container myapp-backend -- only possible if container is stopped
$ docker rm myapp-backend
myapp-backend

// `docker ps -a` now shows no container refrenced
$ docker ps -a
CONTAINER ID   IMAGE     COMMAND   CREATED   STATUS    PORTS     NAMES

// image can be successfully removed when no container is refrencing the image
$ docker rmi myapp:local
Untagged: myapp:local
Deleted: sha256:ec2be4ef4307d65aceb931586183105f1ed9537448db6b18558383af9a422663
Deleted: sha256:89452ae547988b560a19473531cda7e55b04d9d34aa82a64780bce22f561c314
Deleted: sha256:f4175ef030810f3647908804414a9155668a9ebb74103c841886d087c999340f
Deleted: sha256:086485045b5f8766321d6002b2dba8b6ee5ddec6770fdcf23e684dfd3166e431
Deleted: sha256:232fab3b6c7e9e6d7c3f7375bb3143ad5678a901d259228d104ab2a0c6d41c23
Deleted: sha256:341305565a59d36f93f5edd507e3ea5f58eb393ea190c0baced61fd3a8ba1398
Deleted: sha256:9df867464039faa8eebe54a8cc77e2f341613ca37ad601008c4b4216eee6c701
Deleted: sha256:08c45221a1cf843bb6fec6b69f6573b8c05737728730fba0916fae3d4f5c67d4
Deleted: sha256:43f7baff39c3f938d19119029caad63a0fabc466c43fa9472b7894b6153942a2
Deleted: sha256:ae0fbfcdfa70a5b6f6b31867ad9a22cbf4a4dfb6ce28077f31eb1ea88680ca60
Deleted: sha256:2e463b829a56f5f3a69c181b83f2cbb2a56b90df555c213716d5b3811f956be1
Deleted: sha256:cb77dbe80adb5684fcee23d66bc899144981bc872391747f5a28e536a50d4544
Deleted: sha256:f796d3d2f8f2f8ef58906298c82b320f4f3139c377dd76c60457ba11e17fd854
Deleted: sha256:e30ce81b4172b4bbe85d45ab2863986ac77a511d8f5b2ee52c27347617865f6a
Deleted: sha256:5ff83b5d3260c90aedc2781a761c6c1acbb82458be88f092b15788eafcb35db4
Deleted: sha256:e8bce0aabd687e9ee90e0bada33884f40b277196f72aac9934357472863a80ae
$

// below commands shows local docker
// no images
// no container process running
// no referenced docker images
$ docker images
REPOSITORY   TAG       IMAGE ID   CREATED   SIZE
$
$ docker ps
CONTAINER ID   IMAGE     COMMAND   CREATED   STATUS    PORTS     NAMES
$
$
$ docker ps -a
CONTAINER ID   IMAGE     COMMAND   CREATED   STATUS    PORTS     NAMES
$
$

new commands​

docker events  --- see docker restarts, kills, exits in real time. <br/>
docker logs -f gnostex-backend -- equivalent of tail -f logs <br/>
docker run -d --rm  --name gnostex-backend-api --add-host=host.docker.internal:host-gateway  -p 8080:8080  -e SPRING_PROFILES_ACTIVE=dev  -e DB_URL=jdbc:postgresql://host.docker.internal:5432/gnostexdb  -e DB_USERNAME=gnostex  -e DB_PASSWORD=gnostex  gnostex-backend-api <br/>
--add-host=host.docker.internal:host-gateway -- Map host.docker.internal to the Linux host<br/>
try running postgresql also as a container <br/>
docker build -t gnostex-backend-api . -- docker build <br/>
docker run -it --rm  --entrypoint sh  gnostex-backend-api  -- for exploring the image<br/>
curl -f http://localhost:8080/actuator/health

{"status":"UP","components":{"db":{"status":"UP","details":{"database":"PostgreSQL","validationQuery":"isValid()"}},"diskSpace":{"status":"UP","details":{"total":206900281344,"free":195738656768,"threshold":10485760,"path":"/app/.","exists":true}},"ping":{"status":"UP"},"ssl":{"status":"UP","details":{"validChains":[],"invalidChains":[]}}}}

docker inspect
docker inspect --format='{{.State.Health.Status}}' gnostex-backend-api