BLD: updated CI and integration with docker-environment

[lacabra]

This is an opinionated PR that restructures how this repo integrates with docker-environment, and how the CI runs.

• Like all other repos in the Enigma Protocol (see core, contract and p2p, the definition and building of docker images is externalized and centralized into the docker-environment repo.
• Local copies of docker-compose.yml have been updated from the latest copy in docker-environment
• In the CI, the images for salad_operator and salad_client are built from docker-environment using the current branch from this repo. Because Github CI does not provide the BRANCH environment variable, we have to parse it from the available $GITHUB_REF (see their documentation

[reuvenpo]

I believe that just using $GITHUB_REF without modification should work too, but we need to check.