Add Docker install on Ubuntu

src/docker.md

@@ -6,6 +6,7 @@
 # Add the current user to the docker group so docker can be run without sudo.
 sudo usermod -aG docker ${USER}
 su - ${USER}
+exit
 ```
 
 ## Basics
@@ -71,3 +72,27 @@ docker stats
 docker image ls --filter dangling=true
 docker image prune
 ```
+
+## Install on Ubuntu
+On Ubuntu, from https://docs.docker.com/engine/install/ubuntu/:
+
+```bash
+sudo apt-get update
+sudo apt-get install ca-certificates curl gnupg
+
+sudo install -m 0755 -d /etc/apt/keyrings
+curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo gpg --dearmor -o /etc/apt/keyrings/docker.gpg
+sudo chmod a+r /etc/apt/keyrings/docker.gpg
+
+echo \
+  "deb [arch="$(dpkg --print-architecture)" signed-by=/etc/apt/keyrings/docker.gpg] https://download.docker.com/linux/ubuntu \
+  "$(. /etc/os-release && echo "$VERSION_CODENAME")" stable" | \
+  sudo tee /etc/apt/sources.list.d/docker.list > /dev/null
+
+sudo apt-get update
+
+sudo apt-get install docker-ce docker-ce-cli containerd.io docker-buildx-plugin docker-compose-plugin
+
+# Test.
+sudo docker run hello-world
+```