Work around tox-docker KeyError on Docker 29+

[jonathangreen]

Description

Added TOX_DOCKER_GATEWAY environment variable to the test workflow to work around a KeyError: 'Gateway' failure in tox-docker when running on Docker 29+.

Docker 29 removed the deprecated top-level NetworkSettings.Gateway key from the container inspect API. tox-docker 5.0.0 relies on this key and crashes with a KeyError. Setting TOX_DOCKER_GATEWAY=0.0.0.0 bypasses the broken lookup entirely.

This became flaky starting Feb 12 as the Docker 29 rollout to GitHub Actions runners progresses. Once complete, tests will fail 100% of the time without this fix.

This workaround can be removed once tox-docker merges the fix.

Mirrors the fix applied in circulation#3045 and library-registry#862.

Motivation and Context

CI test runs are failing intermittently with:

tox_docker/plugin.py, line 53, in get_gateway_ip
    ip = container.attrs["NetworkSettings"]["Gateway"] or "0.0.0.0"
KeyError: 'Gateway'

How Has This Been Tested?

• CI will validate that tests pass with the workaround on Docker 29+ runners

Checklist

• I have updated the documentation accordingly.
• All new and existing tests passed.