See this page for more details on how to install Docker Engine for Ubuntu: https://docs.docker.com/install/linux/docker-ce/ubuntu/

• Check if Docker was already installed:
  

  $ dpkg -l | grep docker

  docker-ce/bionic,now 5:19.03.12~3-0~ubuntu-bionic amd64 [installed]
  docker-ce-cli/bionic,now 5:19.03.12~3-0~ubuntu-bionic amd64 [installed]

  $ dpkg -l | grep containerd

  containerd.io/bionic,now 1.2.13-2 amd64 [installed]

  You might have Docker installed with different package names: docker, docker.io , or docker-engine
  
  
• To uninstall an existing installation of Docker:
  

  $ for pkg in docker.io docker-doc docker-compose docker-compose-v2 podman-docker containerd runc; do sudo apt-get remove $pkg; done

  [Optional] Delete Docker data folder:
  

  $ sudo rm -rf /var/lib/docker

  [Optional] Delete Docker custom configuration folder:
  

  $ sudo rm -rf /etc/docker/

  [Optional] Delete docker command configuration folder:
  

  $ rm -rf ~/.docker/

• Update the apt package index:
  

  $ sudo apt-get update

• [Optional] Install packages to allow apt to use a repository over HTTPS:
  

  $ sudo apt-get install \
      apt-transport-https \
      ca-certificates \
      curl \
      gnupg-agent \
      software-properties-common

• Add Docker’s official GPG key:
  

  $ sudo apt-get install ca-certificates curl
  $ sudo install -m 0755 -d /etc/apt/keyrings
  $ sudo curl -fsSL https://download.docker.com/linux/ubuntu/gpg -o /etc/apt/keyrings/docker.asc
  $ sudo chmod a+r /etc/apt/keyrings/docker.asc

• Set up the repository:
  

  $ echo \
    "deb [arch=$(dpkg --print-architecture) signed-by=/etc/apt/keyrings/docker.asc] https://download.docker.com/linux/ubuntu \
    $(. /etc/os-release && echo "${UBUNTU_CODENAME:-$VERSION_CODENAME}") stable" | \
    sudo tee /etc/apt/sources.list.d/docker.list > /dev/null

• Update the apt package index:
  

  $ sudo apt-get update

• Install the latest version of Docker Engine and containerd:
  

  $ sudo apt-get install docker-ce docker-ce-cli containerd.io docker-buildx-plugin docker-compose-plugin

• Check Docker daemon (dockerd server):
  

  $ ps -ef | grep docker

  root    12269    /usr/bin/dockerd -H fd:// --containerd=/run/containerd/containerd.sock

• Check Docker service:
  

  $ sudo systemctl status docker

  ● docker.service - Docker Application Container Engine
       Loaded: loaded (/lib/systemd/system/docker.service; enabled; vendor preset: enabled)
       Active: active (running) since Tue 2024-04-29 06:59:47 EDT; 19min ago
  TriggeredBy: ● docker.socket
         Docs: https://docs.docker.com
     Main PID: 9987 (dockerd)
        Tasks: 13
       Memory: 21.9M
          CPU: 418ms
       CGroup: /system.slice/docker.service
               └─9987 /usr/bin/dockerd -H fd:// --containerd=/run/containerd/containerd.sock

• Check containerd Service:
  

  $ sudo systemctl status containerd

  ● containerd.service - containerd container runtime
       Loaded: loaded (/lib/systemd/system/containerd.service; enabled; vendor preset: enabled)
       Active: active (running) since Tue 2024-04-29 06:59:46 EDT; 19min ago
         Docs: https://containerd.io
     Main PID: 9834 (containerd)
        Tasks: 12
       Memory: 13.7M
          CPU: 886ms
       CGroup: /system.slice/containerd.service
               └─9834 /usr/bin/containerd

$ sudo docker version

Client: Docker Engine - Community
 Version:           28.1.1
 API version:       1.49
 Go version:        go1.23.8
 Git commit:        4eba377
 Built:             Fri Apr 18 09:52:10 2025
 OS/Arch:           linux/amd64
 Context:           default

Server: Docker Engine - Community
 Engine:
  Version:          28.1.1
  API version:      1.49 (minimum version 1.24)
  Go version:       go1.23.8
  Git commit:       01f442b
  Built:            Fri Apr 18 09:52:10 2025
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          1.7.27
  GitCommit:        05044ec0a9a75232cad458027ca83437aae3f4da
 runc:
  Version:          1.2.5
  GitCommit:        v1.2.5-0-g59923ef
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0

Note the versions of the client and the server.
Also note the version of the API version supported by the server (and the minimum compatible version).

$ sudo docker system info

Client: Docker Engine - Community
 Version:    28.1.1
 Context:    default
 Debug Mode: false
 Plugins:
  buildx: Docker Buildx (Docker Inc.)
    Version:  v0.23.0
    Path:     /usr/libexec/docker/cli-plugins/docker-buildx
  compose: Docker Compose (Docker Inc.)
    Version:  v2.35.1
    Path:     /usr/libexec/docker/cli-plugins/docker-compose

Server:
 Containers: 0
  Running: 0
  Paused: 0
  Stopped: 0
 Images: 0
 Server Version: 28.1.1
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Using metacopy: false
  Native Overlay Diff: true
  userxattr: false
 Logging Driver: json-file
 Cgroup Driver: systemd
 Cgroup Version: 2
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local splunk syslog
 Swarm: inactive
 Runtimes: io.containerd.runc.v2 runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 05044ec0a9a75232cad458027ca83437aae3f4da
 runc version: v1.2.5-0-g59923ef
 init version: de40ad0
 Security Options:
  apparmor
  seccomp
   Profile: builtin
  cgroupns
 ID: 6bf6b3bf-947e-4048-889a-8391e7164a8e
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 Experimental: false
 Insecure Registries:
  ::1/128
  127.0.0.0/8
 Live Restore Enabled: false

The command might show warnings with your Docker installation if any.

The command docker system provides more options:

$ docker system --help

Usage:  docker system COMMAND

Manage Docker

Commands:
  df          Show docker disk usage
  events      Get real time events from the server
  info        Display system-wide information
  prune       Remove unused dat

You can print the Docker disk usage:

$ docker system df

TYPE            TOTAL     ACTIVE    SIZE      RECLAIMABLE
Images          24        9         2.534GB   1.681GB (66%)
Containers      35        18        831.5kB   413.7kB (49%)
Local Volumes   0         0         0B        0B
Build Cache     10        0         12.29kB   12.29kB

The command "docker container run hello-world" downloads the hello-world image and runs it in a container.
When the container runs, it prints a "Hello from Docker!" message and exits.

$ sudo docker container run hello-world

Unable to find image 'hello-world:latest' locally
latest: Pulling from library/hello-world
e6590344b1a5: Pull complete
Digest: sha256:c41088499908a59aae84b0a49c70e86f4731e588a737f1637e73c8c09d995654
Status: Downloaded newer image for hello-world:latest

Hello from Docker!
This message shows that your installation appears to be working correctly.

To generate this message, Docker took the following steps:
 1. The Docker client contacted the Docker daemon.
 2. The Docker daemon pulled the "hello-world" image from the Docker Hub.
    (amd64)
 3. The Docker daemon created a new container from that image which runs the
    executable that produces the output you are currently reading.
 4. The Docker daemon streamed that output to the Docker client, which sent it
    to your terminal.

To try something more ambitious, you can run an Ubuntu container with:
 $ docker container run -it ubuntu bash

Share images, automate workflows, and more with a free Docker ID:
 https://hub.docker.com/

For more examples and ideas, visit:
 https://docs.docker.com/get-started/

• To Enable/Disable Docker service (start on boot):
  

  $ sudo systemctl enable docker
  $ sudo systemctl disable docker

• To Start/Stop Docker service:
  

  $ sudo systemctl start docker
  $ sudo systemctl stop docker

• Important folders/files:
  /var/lib/docker/
  /etc/docker/key.json
  /etc/docker/daemon.json
  /lib/systemd/system/docker.service
  /lib/systemd/system/docker.socket
  /lib/systemd/system/containerd.service
  
  /etc/docker/daemon.json: might not be created by default when you install Docker. For Docker Desktop, you need to use the its UI to modify this file. It can hold configurations related to the dockerd server, including the arguments we use when we start Docker (e.g., "experimental", "metrics-addr")

  $ sudo ls -1 /var/lib/docker/

  buildkit
  containers
  engine-id
  image
  network
  overlay2
  plugins
  runtimes
  swarm
  tmp
  volumes

  $ cat /lib/systemd/system/docker.service

  [Unit]
  Description=Docker Application Container Engine
  Documentation=https://docs.docker.com
  After=network-online.target nss-lookup.target docker.socket firewalld.service containerd.service time-set.target
  Wants=network-online.target containerd.service
  Requires=docker.socket
  
  [Service]
  Type=notify
  ExecStart=/usr/bin/dockerd -H fd:// --containerd=/run/containerd/containerd.sock
  ExecReload=/bin/kill -s HUP $MAINPID
  TimeoutStartSec=0
  RestartSec=2
  Restart=always

  $ cat /lib/systemd/system/docker.socket

  [Unit]
  Description=Docker Socket for the API
  
  [Socket]
  ListenStream=/run/docker.sock
  SocketMode=0660
  SocketUser=root
  SocketGroup=docker

  $ cat /lib/systemd/system/containerd.service

  [Unit]
  Description=containerd container runtime
  Documentation=https://containerd.io
  After=network.target local-fs.target dbus.service
  
  [Service]
  ExecStartPre=-/sbin/modprobe overlay
  ExecStart=/usr/bin/containerd