Allow specifying GATEWAY_IP via environment variable

[thenewguy]

Environments using docker in docker (for example, tests executing via Gitlab CI runners) need to use a different host to reach the container than the gateway ip currently being chosen.

This isn't easily detectable so allow setting the environment variable TOX_DOCKER_GATEWAY_IP to override the address returned by _get_gateway_ip

It would also be convenient in some cases to specify the gateway as a hostname instead of ip. Perhaps, TOX_DOCKER_GATEWAY_HOST.

It should be an error to provide both TOX_DOCKER_GATEWAY_IP and TOX_DOCKER_GATEWAY_HOST.

Here is an example gitlab ci job demonstrating where it would be more convenient to set the gateway as a hostname like TOX_DOCKER_GATEWAY_HOST: 'docker' instead of by the ip:

test_tox:
  image: docker:19.03
  services:
    - docker:19.03-dind
  script:
    - export TOX_DOCKER_GATEWAY_IP=$(getent hosts docker | cut -d' ' -f1)
    - tox -vv -e py3

[Dan Crosta]

Closed by #54