Allow manual image builds on master

After fixing a broken master caused by a dependency updated that introduced a regression in markdownlint, I realized we should probably put the image build jobs into the master pipeline, as manual jobs. This will let us rebuild images after we updated a dockerfile, by clicking on the play button once it's merged to master. Previously (and just now), we'd need to trigger a sheduled job to do the same, so this will save time/resources.