initial implementation

Provide an initial implementation of this module. Post-merge, a new v1 branch needs creating.

This is based on earlier work within the Apigee Ops configuration. Not much has changed but we now refer to the source image explicitly by id when pushing to handle corner cases where images are re-tagged as the terraform deployment is running. (Ask me how I know...)

Part of https://gitlab.developers.cam.ac.uk/uis/devops/raven/infrastructure/-/issues/35