[BuildStream] Move Docker source plugin out of bst-external

[Chiara Tolentino <ctolentino3891 gmail com>]

(Forking off the discussion in [1] as this should be two separate
conversations really)

During the last BuildStream Gathering, it was generally agreed that the Docker
source plugin should be moved out of bst-external to a new containers
repository. While the concerns regarding how CI would be run for these separate
plugin repositories are valid and must be addressed, bst-external already has
the issue of having to copy-paste tests from core to be able to run them. The
new containers repository would then be a good place to start experimenting on
how CI would eventually work for plugin repositories.

With this change, it's expected that users of the Docker source plugin would
then have to install the migrated plugins separately (either from PyPI or,
as in the case of bst-external currently, by cloning the repository). And as
this only concerns bst-external, this would not affect buildstream/buildstream
or the upcoming v1.4 release in any way.

Please let me know your thoughts and ideas on this.

Cheers,
Chiara

[1]: https://mail.gnome.org/archives/buildstream-list/2018-October/msg00057.html