[BuildStream] BuildStream Docker images - updates and removals

[Chandan Singh <chandan chandansingh net>]

Hi,

tl;dr
=====

* If you are using `contrib/bst-here` script, update your local copy of it.
* If you are using `buildstream/buildstream-fedora` image elsewhere, switch to
  `buildstream/buildstream` image.
* If you are using any of the `buildstream/testsuite-*` images (from Docker Hub)
  switch to the corresponding images from
  `registry.gitlab.com/buildstream/buildstream-docker-images`.

What's changing
===============

Previously all Docker images managed by the buildstream-docker-images
repository used to be published on Docker Hub. Since that is the public face of
BuildStream's presence on Docker Hub, it is important to keep that clean, and
only put images there that we expect end-users to use.

To that end, the `testsuite-*` that are used in BuildStream's CI are no longer
published on Docker Hub. They will only be published on GitLab's container
registry. You can browse these images at
https://gitlab.com/BuildStream/buildstream-docker-images/container_registry.

The primary user-facing `buildstream/buildstream-fedora` is now deprecated.
Instead, users should switch to `buildstream/buildstream` image. In near
future, using this image will result in warnings. And in one month's time, it
will stop receiving any updates, and will be removed entirely after three
months.

The new image comes in several variants. While `latest` tag will continue to
track the latest stable release of BuildStream (i.e. 1.2.x series at present),
it has never been easier to try new versions of BuildStream! You can simply run
`bst-here -j nightly` to try the `master` branch of BuildStream, or
`bst-here -j dev` to try the latest development snapshot of
BuildStream (i.e. 1.3.x series).

There are even more options available, and you can find out more about the
image variants at http://hub.docker.com/r/buildstream/buildstream.

What do I need to do
====================

The main BuildStream repository has already been updated accordingly, so no
changes are needed there. If you are using these images elsewhere, look at the
tl;dr section at the top.

Relevant dates
==============

In one month's time, on 18th March 2019:

* `buildstream/buildstream-fedora` will stop receiving any updates.

* The following testsuite images will be REMOVED from Docker Hub:

    - buildstream/testsuite-debian
    - buildstream/testsuite-fedora
    - buildstream/testsuite-centos
    - buildstream/testsuite-ubuntu
    - buildstream/testsuite-ubuntu-18
    - buildstream/testsuite-python
    - buildstream/buildstream-debian

In three months' time, on 18th May 2019:

* `buildstream/buildstream-fedora` will be REMOVED from Docker Hub.

If you think that any of the above dates are unreasonable, or if you face any
issues in the migration process, please let me know.


Cheers,
Chandan