Add background cleanup process

When given the "run" argument, in addition to launching `podman system
service` in the background, also start a small periodic maintenance
script.  It's only job is to clean up stale images, containers, and
volumes from old jobs.  Currently hard-coded to trigger every 2 days,
this could be tweaked via build-args or env. var.

Signed-off-by: Chris Evich <cevich@redhat.com>

Containerfile

@@ -86,13 +86,14 @@ RUN if [[ "$RUNNER_LISTEN_ADDRESS" == "disabled" ]]; then \
 
 # A small wrapper is needed to launch a background podman system service
 # process for the gitlab-runner to connect to.
-ADD /gitlab-runner-wrapper /usr/local/bin/
+ADD /gitlab-runner-wrapper /podman-in-podman-maintenance /usr/local/bin/
 # Base image UTS NS configuration causes runner to break when launching
 # nested rootless containers.
 RUN sed -i -r \
         -e 's/^utsns.+host.*/utsns="private"/' \
         /etc/containers/containers.conf && \
     chmod +x /usr/local/bin/gitlab-runner-wrapper && \
+    chmod +x /usr/local/bin/podman-in-podman-maintenance && \
     chown -R podman.podman /home/podman && \
     rm -f /home/podman/.bash* && \
     echo DOCKER_HOST="unix:///tmp/podman-run-1000/podman/podman.sock" > /etc/profile.d/podman.sh