Add DOCKER_EXPERIMENTAL environment variable

The DOCKER_EXPERIMENTAL environment variable drives the activation of
the 'experimental' build tag.

Signed-off-by: Arnaud Porterie <arnaud.porterie@docker.com>
Upstream-commit: ca6722f1c5e4e2017a71e33579d91ac0d9ea2a25
Component: engine

components/engine/Makefile

@@ -7,6 +7,7 @@ DOCKER_ENVS := \
 	-e BUILDFLAGS \
 	-e DOCKER_CLIENTONLY \
 	-e DOCKER_EXECDRIVER \
+	-e DOCKER_EXPERIMENTAL \
 	-e DOCKER_GRAPHDRIVER \
 	-e DOCKER_STORAGE_OPTS \
 	-e TESTDIRS \