ci: Restrict docker builds to main branch

If we push feature branches to yocto-ktn a build of the docker images is triggered and new images are pushed to the registry. We only want this to happen for changes on the main branch (master).

In the future we should separate the build and deploy stage for the docs and the docker images to make sure we run the build part for all branches (to do build testing) and only restrict the push/deploy to the main branch.