docker-compose: add 'mender' network
docker-compose: add 'mender' network

Signed-off-by: Maciej Borzecki <f609d9f713abcd5f8f0e51f08dc151638a7681e7@open-rnd.pl>

pasinskim/integration,pasinskim/integration,GregorioDiStefano/integration,GregorioDiStefano/integration,pasinskim/integration