.gitlab-ci.yml: Make docker image name more explicit

.gitlab-ci.yml: Make docker image name more explicit

The 'image' YAML tag implies a docker image, but for people who find
gitlab mysterious, let's make it blatantly obvious what we're doing
here.

+ added a comment
+ added 'DOCKER' to the variable names
+ removed 'BUILD', as we've now dropped this from all the job-names
+ tried to make the variable names consistent, both within the file and
WRT docker terminology

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Thu Feb 14 03:51:21 CET 2019 on sn-devel-144