Setup a production image workflow in CI

[daanboer]

This workflow will build the production docker images required to run LXCat. This workflow does not need to run on every pull request, but can e.g. be ran periodically or on making a new release. It would be nice to make the release version visible on the website, such that e.g. bug reports can always report a specific platform version, and we can tie this version to e.g. the schema version that is used (see related #177). The LXCat version can be communicated to the build process through an environment variable.