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>

podman-in-podman-maintenance

@@ -0,0 +1,19 @@
+#!/bin/bash
+
+# This script is intended to be called by the entrypoint for
+# a podman-in-podman gitlab runner container.  Any usage
+# outside that context is not supported and may cause harm.
+
+set -e
+
+maintain_podman() {
+  # Two days seems to be a good happy-medium beween filling up
+  # about 40gig of storage space from moderate CI activity,
+  # and maintaining a useful level of caching.
+  while sleep 2d; do
+    if [[ -n "$PODMAN_RUNNER_DEBUG" ]]; then
+      echo "$(date --iso-8601=second) ${BASH_SOURCE[0] performing podman maintenance}"
+    fi
+    podman system prune --all --force
+  done
+}