Create new buildstream/buildstream image

Follow-up from #17 (comment 92323965).

The real user-facing image we currently offer is buildstream/buildstream-fedora. This is not ideal as it leaks info about our base distro in the name of the image, and will prevent us from changing the base of our image if needed. Also, this is more for the users to type.

Similar to other projects, we should name our image buildstream/buildstream. If needed, we can use tags to designate different flavors/variants instead of the name. For example, buildstream/buildstream:{slim,extra,alpine,...}.

Here's what we need to do:

• Create a new buildstream/buildstream image in this repo with the following tags: (!107 (merged))

  • latest: tracks latest stable release, i.e. latest even minor point release
  • dev: tracks latest development snapshot, i.e. latest odd minor point release
  • nightly: tracks master of the BuildStream repo

• Update bst-here to use the new image. (buildstream!1153 (merged))

• Announce the release of the new image and deprecation of buildstream/buildstream-fedora on mailing list. Also, propose and agree on a decommissioning date when buildstream/buildstream-fedora will be removed from Docker Hub. Note the deprecation in this context means that the image will stop receiving updates and will be completely removed eventually. (See https://mail.gnome.org/archives/buildstream-list/2019-February/msg00047.html)

• Mark buildstream/buildstream-fedora as deprecated using ENTRYPOINT etc. This will be the final update to this image. (!110 (merged))

• Remove buildstream/buildstream-fedora on the decommissioning date. (Captured in #13 (closed))

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Additionally, we can create two more variants of each image:

• slim: without any plugin base dependencies (like git, ostree etc). This will also solve #14 (closed).

• extra: with certain external plugins installed, like bst-external. This will also be an appropriate way to fix #15 (closed)

This would result in 9 (3x3) variants in total, like :dev-slim, :latest-extra but managing those shouldn't be an issue once #27 (closed) is finished.