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>
Upstream-commit: f672a963c9f1225daec9d2d12577f95936dee51e
Component: engine
2a13e97eec