Create docker push workflow

[indykoning]

This will automatically push docker images into the github container repository under the organisation

[zachgoll]

@indykoning this is great, but our Dockerfile is not quite ready and we'll want to make sure that once we start pushing images to GHCR, all images work.

I think we can either close this out and come back to it later or make the Dockerfile updates directly in this PR

Ultimately, I think we're a bit early with this step as we still have lots of code to write which may affect services packed into the Dockerfile

[zachgoll]

Closing out, will reference this when we introduce official support for a Dockerfile.