Add automated checking of skopeo-synced images

[sd109]

Once #5 is merged, the charts in this repo will use StackHPC's ghcr mirror for container images. This means that bumping the version of an upstream chart (e.g. jupyterhub) may lead to a situation where newer container images which are required by the chart are not yet synced into ghcr. We should be able automate/catch this issue by running helm template on the chart and then parsing the output for container images which can then be added to the skopeo manifests. It may even be possible to extend the existing skopeo workflow to completely automate the generation of skopeo manifests by parsing the output of helm template.