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/entrypoint.sh

@@ -0,0 +1,21 @@
+#!/bin/sh
+set -eu
+
+# Add coder to the group that owns the mounted Docker socket, then restart
+# under that group.
+if [ -S /var/run/docker.sock ]; then
+  gid=$(stat -c %g /var/run/docker.sock)
+  if ! id -G | tr ' ' '\n' | grep -qx "$gid"; then
+    group=$(getent group "$gid" | cut -d: -f1)
+    if [ -z "$group" ]; then
+      group=docker-host
+      sudo groupadd -g "$gid" "$group"
+    fi
+    sudo usermod -aG "$group" coder
+    exec sudo -E setpriv --reuid=coder --regid=coder --init-groups \
+      env PATH="$PATH" USER=coder /usr/bin/entrypoint.sh "$@"
+  fi
+fi
+
+# Start the image's own entrypoint.
+exec /usr/bin/entrypoint.sh "$@"