Build on side branches

[FabijanC]

Currently we only build on master, but for testing purposes, we could build on side branches, too. This is useful if we aren't just doing a simple cairo-lang update, but e.g. adding new commands.

It could be a simple build of just one image. Don't know how we would handle the environment variables (ARGs), ideally we wouldn't introduce duplication.

EDIT: the simplest approach would be to add a check in the shell script which checks the CIRCLE_BRANCH (an environment variable present in circleci) and only pushes the images if on master.