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/Dockerfile

@@ -9,13 +9,34 @@ RUN apt-get update \
     libffi-dev libssl-dev libyaml-dev zlib1g-dev \
   && rm -rf /var/lib/apt/lists/*
 
-# zsh as the container user's login shell.
-RUN chsh -s /usr/bin/zsh coder
+# Docker CLI with the Compose and Buildx plugins, from Docker's apt repository,
+# and GitHub CLI, from GitHub's apt repository.
+RUN install -m 0755 -d /etc/apt/keyrings \
+  && curl -fsSL https://download.docker.com/linux/debian/gpg \
+    -o /etc/apt/keyrings/docker.asc \
+  && curl -fsSL https://cli.github.com/packages/githubcli-archive-keyring.gpg \
+    -o /etc/apt/keyrings/githubcli-archive-keyring.gpg \
+  && chmod a+r /etc/apt/keyrings/docker.asc /etc/apt/keyrings/githubcli-archive-keyring.gpg \
+  && printf '%s\n' \
+    'Types: deb' \
+    'URIs: https://download.docker.com/linux/debian' \
+    "Suites: $(. /etc/os-release && echo "$VERSION_CODENAME")" \
+    'Components: stable' \
+    "Architectures: $(dpkg --print-architecture)" \
+    'Signed-By: /etc/apt/keyrings/docker.asc' \
+    > /etc/apt/sources.list.d/docker.sources \
+  && echo "deb [arch=$(dpkg --print-architecture) signed-by=/etc/apt/keyrings/githubcli-archive-keyring.gpg] https://cli.github.com/packages stable main" \
+    > /etc/apt/sources.list.d/github-cli.list \
+  && apt-get update \
+  && apt-get install -y --no-install-recommends \
+    docker-ce-cli docker-buildx-plugin docker-compose-plugin gh \
+  && rm -rf /var/lib/apt/lists/*
 
 # Workspace directory, owned by the container user.
 RUN mkdir -p /workspace && chown 1000:1000 /workspace
 
-# Startup wrapper: joins the Docker socket's group, then runs the image's entrypoint.
+# Startup wrapper: enters DEFAULT_WORKSPACE, joins the Docker socket's group,
+# then runs the image's entrypoint.
 COPY entrypoint.sh /usr/local/bin/entrypoint.sh
 USER 1000