[BuildStream] End-user docker images use now the latest bst recommended version, not master

[Javier Jardón <jjardon gnome org>]

Hi,

Only to let you know; we have recently fixed a bug where we were
putting buildstream master in the docker images meant to be used for
final users, instead the latest recommended release

The images used for testing in the CI still include bst master
dependencies, of course

See [1] for details

Cheers,
Javier

[1] https://gitlab.com/BuildStream/buildstream-docker-images/merge_requests/72