feat(code-server): install Docker CLI and gh from vendor apt repos

- Install the Docker CLI with its Compose and Buildx plugins, and the GitHub
  CLI, from Docker's and GitHub's signed apt repositories; drop their home
  installs and the GitLab CLI from the README.
- Select the shell with SHELL and the start folder with DEFAULT_WORKSPACE
  instead of chsh and working_dir.
- Add an optional CODE_SERVER_APP_NAME.
- Document installing Java with SDKMAN.

code-server/entrypoint.sh

@@ -1,6 +1,11 @@
 #!/bin/sh
 set -eu
 
+# Start in DEFAULT_WORKSPACE, the folder code-server opens.
+if [ -n "${DEFAULT_WORKSPACE:-}" ]; then
+  cd "$DEFAULT_WORKSPACE"
+fi
+
 # Add coder to the group that owns the mounted Docker socket, then restart
 # under that group.
 if [ -S /var/run/docker.sock ]; then