Build: pass through DOCKER_GITCOMMIT

This is useful if you want to build Docker when the .git directory is
not present.

Signed-off-by: Thomas Leonard <thomas.leonard@docker.com>

Makefile

@@ -18,6 +18,7 @@ DOCKER_ENVS := \
 	-e DOCKER_CLIENTONLY \
 	-e DOCKER_DEBUG \
 	-e DOCKER_EXPERIMENTAL \
+	-e DOCKER_GITCOMMIT \
 	-e DOCKER_GRAPHDRIVER=$(DOCKER_GRAPHDRIVER) \
 	-e DOCKER_INCREMENTAL_BINARY \
 	-e DOCKER_REMAP_ROOT \