Import image-tag script into build tools so it can be shared
Import image-tag script into build tools so it can be shared

We have divergent versions of this script across repos.
This particular version comes from 6334836@scope.
We in particular want this version's changes to support branches
with a '/' character in them.

paulbellamy/scope,kinvolk/scope,kinvolk/scope,alban/scope,weaveworks/scope,kinvolk/scope,weaveworks/scope,weaveworks/scope,alban/scope,paulbellamy/scope,alban/scope,paulbellamy/scope,alban/scope,weaveworks/scope,alban/scope,paulbellamy/scope,paulbellamy/scope,kinvolk/scope,weaveworks/scope,kinvolk/scope,weaveworks/scope,kinvolk/scope,paulbellamy/scope,alban/scope