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>

gitlab-runner-wrapper

@@ -1,5 +1,9 @@
 #!/bin/bash
 
+# This script is intended to be called as the entrypoint for
+# a podman-in-podman gitlab runner container.  Any usage
+# outside that context is not supported and may cause harm.
+
 set -e
 
 unset _debug_args
@@ -9,7 +13,8 @@ fi
 
 if [[ "$1" == "run" ]] && [[ ! -S "/tmp/podman-run-1000/podman/podman.sock" ]]; then
     podman $_debug_args system service -t 0 &
-    # Prevent SIGHUP propigation to podman process
+    /usr/local/bin/podman-in-podman-maintenance &
+    # Prevent SIGHUP propagation to podman process
     disown -ar
 fi