While working on the building automation in #73 it become clear that the current structure of the repo was becoming a bit unwieldy. This PR is a proposal to simplify and consolidate various things.

TL;DR:

• One single place to define versions
• 8 Makefiles -> 1 bash script
• No more base/dev folders, base and dev images built from the same Dockerfile

Here's more detail on the changes from less to more potentially controversial (I think):

• Consolidate version handling. We had version numbers all over the place and many different variables with slightly different meanings. Now we only have the version defined in one place (VERSION.txt), and the vars have been consolidated (Dockerfiles only use CKAN_REF, and the ones in the shell script should be hopefully named as what they are)

• Centralize building (and pushing). Rather than 8 mostly identical but not quite Makefiles we now only have one bash script at the root of the repo (ckan-docker-base.sh) which builds the relevant images according to the passed parameters, e.g.:

  ./ckan-docker-base.sh build 2.11
  
  ./ckan-docker-base.sh build master
  ./ckan-docker-base.sh build 2.10 base
  ./ckan-docker-base.sh build 2.0 dev
  

  It also centralizes commands for pushing the images to DockerHub, but that will be soon done via GitHub Actions so it's discouraged. Of course the bash script is a bit more complex than all the individual Makefiles but I think it's easy enough to follow and extend (please do shout if you think it's not!)

• Reduce number of Dockerfiles and remove the base and dev folders. While we still keep separate images for prod and dev environments (I personally think this is good), they are all now built from the same Dockerfile using staged builds and the Buildkit ability to just build the stages needed to build the final image (so buildkit must be enabled with DOCKER_BUILDKIT=1 when running docker build). It's easier to understand looking at one of the new Dockerfiles but basically we are not building the "dev" stage unless we pass a build param (ENV=dev).

With these changes we end up with this (per CKAN version):

ckan-X.XX
├── Dockerfile          # Dockerfile for the image (in older versions there might be one for alpine and one for python)
├── VERSION.txt         # Full CKAN version built (eg. 2.11.0, 2.10.5)
├── PYTHON_VERSION.txt  # Python version used (eg. 3.10)
└── setup               # Setup scripts used by the images
    └── ...

@wardi @kowh-ai It would be great if you could have a look and see if there's something you are not happy with. To implement the automations in #73 we need to settle on a repo structure. I'm happy to wait until #80 is merged and port the changes here myself if that will happen soon.