ci: Use an init process to reap defunct processes.

When Github Actions run in Docker, the default pid 1 entrypoint is
`tail -f /dev/null`.  PID 1 is responsible for propagating signals to
its children, and calling `waitpid()` on defunct processes; `tail`
does not do these things.  This results in zombie processes piling up
inside the container, which is not an issue in most contexts.

However, it affects `start-stop-daemon`, which hangs when stopping
daemon processes, as they are never reaped.  This appears in CI as
`/etc/init.d/supervisor restart` never being able to succeed.

Run the docker container with `--init`, which spawns a
`/sbin/docker-init` PID 1 to handle the job of an init process.

.github/workflows/production-suite.yml

@@ -133,7 +133,9 @@ jobs:
             os: bullseye
 
     name: ${{ matrix.name  }}
-    container: ${{ matrix.docker_image }}
+    container:
+      image: ${{ matrix.docker_image }}
+      options: --init
     runs-on: ubuntu-latest
     needs: production_build