Green builds for non-committers

When a PR is created by an third party (either Dependabot or a user), the builds fail.

Reason is that docker containers are build *and pushed*. Of course non-committers do not have push access on the registry, so these builds will fail.

Let's tune the build script so it builds without pushing for non-committers. As a minimum this should work for PR's created by Dependabot.