Allow to create docker network bridge dynamically

[NotBad4U]

We must create a local bridge network named "my-net" to run the test suite :
docker network create --driver=bridge --subnet=172.18.0.0/16 my-net
It's because we have to know the ipv4 address backends to register them in the sozu config.toml

testcontainers provide this feature but it's not documented for the moment (issues/509) . They have unit tests to show how to use this feature.