Takes the docker image from concourse/git-resource and modifies the
check/in/out scripts for mirrored clones.
Check will not clone anything, this would be a waste of bandwidth/resources,
instead it checks the remote is valid (by attempting to
git ls-remote) and
returns a new version at a time based on the specified frequency.
This will not track changes to a branch/tag (nor any ref within a repo), instead
this will pass a new "version" onto Concourse.ci in the format
private_key follow the same format as used by
the git-resource, the only new field is
frequency: Optional. How often a new version should be reported
(daily|weekly|monthly), defaults to daily.