Add actions to build and push images to Docker Hub

[amercader]

As discussed in ckan/ckan#8143, we want to automate the publication of artifacts as much as possible, in this case the building and pushing of CKAN Docker images to Docker hub.

This PR adds a new reusable workflow that is call for each CKAN version to build.

The actions work but there's stuff we need to decide before merging.

1. The biggest question is when or how to trigger a build/push, right now these are triggered on every push to this repo, but this is not what we want. Some options:

• When we push some tag(s)
• When there's a new release created
• When we add some labels to a Pull request

2. There are CKAN tags hardcoded in the worflow files, so on every patch release we would need to update these files. There's probably a way to centralize these, reading them from an env var or a file in the repo

[wardi]

is it better to reference the latest ckan-X.Y image instead of a specific point release? IIUC the docker registry lets us update these links, users can get the latest by building and we don't need to update the tags inside the repo on every ckan-docker-base release.

[kowh-ai]

Maybe once a new release has been pushed to DockerHub ie: 2.10.4 or 2.11.0 this remains ‘untouched’ as an image and can be used as a “point in time” release. Subsequent builds of, for instance 2.10 (which actually would be 2.10.4) could be tagged as 2.10.latest. These could be generated weekly.