manibuilder: Automatically create `manibuilder` network bridge

Create a network bridge for all manibuilder containers. We'll add a
service container with access to a local Git repository later.

Change-Id: Idb03c21dbd12bef59a2c683e169015e286c04d11
Signed-off-by: Nico Huber <nico.h@gmx.de>
1 file changed