support/dockker: create and push tagged images

Currently, we refer to the latest version of the image, which means we
can't guarantee any reproducibility. Also, it measn we can't have a
separate images for the maintenance branches (especially the LTS) and
master.

Update the comment in the Dockerfile to create and push tagged images.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>