Propagate BUILD_APT_MIRROR and consume in buil-deb

Passses down BUILD_APT_MIRROR to the docker env.
Ensures BUILD_APT_MIRROR is used when building debs, but only when the
consuming `Dockerfile` actually uses it, otherwise it will cause the
build to fail (e.g. on Ubuntu builds we aren't using APT_MIRROR).

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

Makefile

@@ -11,6 +11,7 @@ DOCKERFILE := $(shell bash -c 'source hack/make/.detect-daemon-osarch && echo $$
 # to allow things like `make KEEPBUNDLE=1 binary` easily
 # `project/PACKAGERS.md` have some limited documentation of some of these
 DOCKER_ENVS := \
+	-e BUILD_APT_MIRROR \
 	-e BUILDFLAGS \
 	-e KEEPBUNDLE \
 	-e DOCKER_BUILD_ARGS \
@@ -60,6 +61,7 @@ DOCKER_PORT_FORWARD := $(if $(DOCKER_PORT),-p "$(DOCKER_PORT)",)
 
 DOCKER_FLAGS := docker run --rm -i --privileged $(DOCKER_ENVS) $(DOCKER_MOUNT) $(DOCKER_PORT_FORWARD)
 BUILD_APT_MIRROR := $(if $(DOCKER_BUILD_APT_MIRROR),--build-arg APT_MIRROR=$(DOCKER_BUILD_APT_MIRROR))
+export BUILD_APT_MIRROR
 
 # if this session isn't interactive, then we don't want to allocate a
 # TTY, which would fail, but if it is interactive, we do want to attach