fully disable container networking
fully disable container networking

apparmor already blocks all network access, but we might as well just tell
docker to disable it completely.

Change-Id: I34af85e2dc9426fd1daa93587b7a5911952cc2a1
Reviewed-on: https://gerrit.instructure.com/60270
Reviewed-by: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
Tested-by: Jenkins

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