fix: ensure dumb-init is PID 1 (coder#4846)

Exec to dumb-init in entrypoint script, so that it can
handle signals and reap subprocesses.

Author: lorenz

ci/release-image/entrypoint.sh

@@ -18,4 +18,4 @@ if [ "${DOCKER_USER-}" ]; then
   fi
 fi
 
-dumb-init /usr/bin/code-server "$@"
+exec dumb-init /usr/bin/code-server "$@"