Add ${DOCKER_TAG} in Docker hooks

Add ${DOCKER_TAG} in Docker hooks

It eases creating Docker images based on branches or tags.

Conflicts:
	docker/components/hooks/post_checkout
	docker/features/hooks/post_checkout
	docker/healthcheck/hooks/post_checkout
	docker/restapi/hooks/post_checkout
	docker/smoke/hooks/post_checkout
	docker/vnf/hooks/post_checkout

Change-Id: Id84513c53cad72a1fcd0debf06579481609b8bc2
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>
(cherry picked from commit 29647efa114151d53ede5ba2d989593992c0b29f)