Build docker images

We don't build images for each compiler, only for latest in series (for now)

All images are built from the same template, Dockerfile.template We install few C -dev dependencies. Make a PR if you need some more.

There's also a generic image without ghc and cabal, which is used in cases where there are no image for that ghc&cabal pair.

make all will build images
make push-images will also push them to the registry
make ghc-8.4.4-image or make generic-image to build single image. This is useful when making changes (and checking the size of resulting images)

Misc

Docker cleanup:

docker system prune --volumes