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/.env.example

@@ -13,3 +13,6 @@ GIT_EMAIL=
 # Container hostname, also passed in as HOST -- the name zsh's prompt shows.
 # Not named HOSTNAME: the deploying shell's own HOSTNAME would override it.
 SERVICE_HOSTNAME=code-server
+
+# Name shown in the title bar and welcome page.
+# CODE_SERVER_APP_NAME=code-server