build/docker: CI: no tag reuse on env var change

build/docker: CI: no tag reuse on env var change

The previous change adding support for OPNFV build tag reuse
added the get/put-cache functionality to `build/docker` just
for detecting env var changes between tag creation and current build.

Obviously, when building outside CI, it is up to the end user to
force a docker rebuild when env vars change (this was the behavior
before the tag reuse support too).

An alternative approach would be removing get/put-cache completely,
if we consider the env vars (http proxy related) never change for
CI cached builds (seems to be true, worst case we wait 1 day for
the tag to expire when it happens).

This change just fixes the original intended behavior in tag reuse.

JIRA: FUEL-204

Change-Id: I14666994bc334012a5198a64a4a24cd4962e4bd6
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>