buildstream/buildstream-fedora image uses BuildStream's master branch

Currently we create the buildstream/buildstream-fedora image in CI. This is image we wish users of the tool to use if they are unable to install BuildStream from source on their host system.

If I have understood the .gitlab-ci.yml file correctly, we clone the BuildStream repo (which defaults to master), copy the directory into the container (as shown in the Dockerfile), and build the buildstream/buildstream-fedora image with this.

Now that BuildStream does not guarantee that master will be stable, should we build this image with a known-to-be stable version of BuildStream?