Build on side branches

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`.

Look at the existing CircleCI configuration and the shell script that builds Docker images. Identify where the build is triggered and where the CIRCLE_BRANCH environment variable can be used. Modify the script to conditionally push images only on the master branch, ensuring no duplication of ARGs. Test by creating a side branch and verifying the build runs without pushing.

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