The entrypoint waited for both processes, so if guacd died the container
kept running with every session failing, and a restart policy never
fired (and the same if rustguac died). Wait for whichever exits first
with bash's wait -n, stop the other, and exit with its status. Tested:
docker stop in 0.3s; killing guacd or rustguac now stops the container.
(cherry picked from commit 30f384e6efa92539c30e80263aa1afd1a21ae370)