Don't use ENV for the frontend-build
Don't use ENV for the frontend-build

The ARG is enough to set the environment for the subsequent commands in
the frontend-build image.

Specifically, this is used in gulpfile.js, where it changes the paths
referenced in CSS (and probably other places) as to where are the static
assets, prepending the /static/ prefix if we're in a *non* development
environment, so that we fetch things from the CDN

alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin