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>

README.md

@@ -154,6 +154,10 @@ Several build arguments are available to control the output image:
   exact podman version.  Possible values include, `latest`, `vX`, `vX.Y`,
   and `vX.Y.Z` (where, `X`, `Y`, and `Z` represent the podman semantic
   version numbers).  It's also possible to specify an image SHA.
+* `CLEAN_INTERVAL` - A `sleep` (command) compatible time-argument that
+  determines how often to clean out podman storage of disused containers and
+  images.  Defaults to 24-hours, but should be adjusted based on desired caching-effect
+  versus available storage space and rate of job execution.
 * `EXCLUDE_PACKAGES` - A space-separated list of RPM packages to prevent
   their existence in the final image.  This is intended as a security measure
   to limit the attack-surface should a gitlab-runner process escape it's