Fix ability to roll back

[kaihendry]

Since we don't fix on a version of our base, i.e. we always use latest with FROM uneet/bugzilla, this make it very difficult to fix versions and roll back.

We should be fixing versions like FROM mozillabteam/bmo-slim:20180225.1 and furthermore deploying by uneet/bugzilla-customisation:${COMMIT}

The work around is to basically roll master back, which is super awkward.

[kaihendry]

We can roll back be specifying a previous tag: https://hub.docker.com/r/uneet/bugzilla-customisation/tags

But note that in some cases, like our Unicode update, will make older versions (that didn't support the collation) run poorly or not at all.

As a rule we need to move forward.