Add docker image publish workflow

[ctkcoding]

Added a workflow to publish docker images to github and dockerhub using the existing Dockerfile.

[sirodoht]

Hey @ctkcoding! I'm not using Docker and I don't have an account at Docker Hub, so I will postpone exploring this further for the time being as I'm not sure how useful it could be :)

Thanks for suggesting it in any case!

[ctkcoding]

I find it really useful to have a docker artifact to pull instead of source code with extra config re subdomains etc like in the ticket that I raised yesterday, and it will probably need a few extra changes to handle that setup. I will take a look at doing that and cut the Docker Hub flow if you don't have an account and leave the part that auto publishes to github's container registry when you create releases which it looks like you're already doing.