Add ghcr.io Docker container generation to workflow actions

[cortices]

There's an open PR for improving the docker image size, which would be great to get merged.

But also it's difficult to deploy to any non-local server or kubernetes without a prebuilt image.

This page from github support has pretty much a ready-to-go example for adding docker image generation to workflows. Until then I'll be having to copy images manually over ssh to my server, which is not ideal.

[Johann150]

As noted in https://github.com/mbrubeck/agate/pull/144#issuecomment-1110073345 I am still waiting for modifications to be made to that pull request. To clarify that I think that pull request is not complete I have marked it as a draft.

As noted in https://github.com/mbrubeck/agate/issues/145#issuecomment-1033561978 I am not knowledgeable in Docker. If you want the thing you mentioned, feel free to open a pull request.

[cortices]

Thanks for the info. For what it's worth, I've used @timwalls' Dockerfile in the past and it's been great, but his repo is now several years behind this one. I am happy to provide a PR that merges his version's more canonical approach to copying source, with the same directory and calling conventions as this repo's one, so as to reduce the likelihood of a breaking change. I will look at what is needed to get github actions to compile the image too.