Fix maintenance task

The function was defined but never called, resulting in immediate exit
of the maintenance script.  Fix this, also add a configuration build-arg and
ENV to control the cleaning interval.

Signed-off-by: Chris Evich <chris_gitlab@icuc.me>

podman-in-podman-maintenance

@@ -4,16 +4,23 @@
 # a podman-in-podman gitlab runner container.  Any usage
 # outside that context is not supported and may cause harm.
 
-set -e
+set -eo pipefail
 
 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
+  while sleep "$CLEAN_INTERVAL"; do
     if [[ -n "$PODMAN_RUNNER_DEBUG" ]]; then
       echo "$(date --iso-8601=second) ${BASH_SOURCE[0] performing podman maintenance}"
     fi
     podman system prune --all --force
   done
 }
+
+if [[ -z "$CLEAN_INTERVAL" ]]; then
+  echo "ERROR: Empty/unset \$CLEAN_INTERVAL"
+  exit 1
+fi
+
+maintain_podman