contrib/bst-here: Allow users to specify image variant

• contrib/bst-here: Change default image to buildstream/buildstream

  See buildstream-docker-images#26 (closed) for detailed discussion around this.

  buildstream/buildstream-fedora is now considered deprecated. Switch to buildstream/buildstream image. This image also offers more tags that will provide users more flexibility.

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• contrib/bst-here: Allow users to specify image variant

  Now that the buildstream/buildstream image has 9 variants, let's make it easier to choose the desired tag, using a command-line option.

  This is otherwise possible by specifying the full image name image:tag using the -i option. But, this will make it easier to specify just the tag using -j.

  The following two invocations of bst-here are now equivalent:

  bst-here -i buildstream/buildstream:dev
  bst-here -j dev