Add no-op Dockerfile to pass CI

We could have also probably just set a flag in the .gitlab-ci.yml file to not run anything for jobs in this project. The advantage of doing it this way is that we do still get a CI pass/fail here so that if we make a change to this file that breaks the docker build/release steps it will complain. The disadvantage as far as I can see it is that we'll have a sort of phantom glue image that will get published to the registry that will be basically empty 🤷