pass DOCKER_CROSSPLATFORMS to the container in the Makefile

Signed-off-by: Victor Vieux <victorvieux@gmail.com>

Makefile

@@ -23,6 +23,7 @@ DOCKER_ENVS := \
 	-e DOCKER_BUILD_ARGS \
 	-e DOCKER_BUILD_GOGC \
 	-e DOCKER_BUILD_PKGS \
+	-e DOCKER_CROSSPLATFORMS \
 	-e DOCKER_DEBUG \
 	-e DOCKER_EXPERIMENTAL \
 	-e DOCKER_GITCOMMIT \