feat(code-server): join the Docker socket's group at startup and add build tools

Add an entrypoint wrapper that reads the socket's GID, adds coder to a
matching group and re-execs the image's entrypoint under it, so docker
works without sudo on any host. Install build-essential and the headers
rbenv needs to build Ruby, and make zsh the login shell. Document home
installs for rbenv, Docker Compose and Buildx plugins, and jq.

code-server/Dockerfile

@@ -4,10 +4,19 @@ USER root
 
 # Extra system packages.
 RUN apt-get update \
-  && apt-get install -y --no-install-recommends bubblewrap unzip zip \
+  && apt-get install -y --no-install-recommends \
+    build-essential bubblewrap unzip zip \
+    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
+
 # 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.
+COPY entrypoint.sh /usr/local/bin/entrypoint.sh
 USER 1000
+
+ENTRYPOINT ["/usr/local/bin/entrypoint.sh", "--bind-addr", "0.0.0.0:8080", "."]