[ci] follow-up to !17: don't do docker builds on merge requests

this currently isn't possible since docker login can only work on protected branches (to protect the value of the DOCKERHUB_TOKEN variable)

it would still be nice to be able to test changes to the docker build in merge requests, and I don't think docker login is strictly necessary for that, but for now it will just be disabled until there's an immediate need for it; changes to the Docker build can also be tested locally, and should not be any different when run from the CI system