Work around tox-docker KeyError on Docker 29+ (#3045)

## 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](https://docs.docker.com/engine/release-notes/29/) 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](actions/runner-images#13474) 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](tox-dev/tox-docker#196).

Mirrors the fix applied in
[library-registry#862](ThePalaceProject/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

- [x] I have updated the documentation accordingly.
- [x] All new and existing tests passed.

Author: jonathangreen

.github/workflows/test.yml

@@ -46,6 +46,13 @@ jobs:
 
       - name: Run Tests
         run: tox
+        env:
+          # Workaround for tox-docker not supporting Docker 29+, which removed
+          # the top-level NetworkSettings.Gateway key from the container API.
+          # This can be removed once tox-docker merges the fix.
+          # Docker 29 rollout: https://github.com/actions/runner-images/issues/13474
+          # tox-docker fix: https://github.com/tox-dev/tox-docker/pull/196
+          TOX_DOCKER_GATEWAY: "0.0.0.0"
 
       - name: Upload coverage to Codecov
         uses: codecov/codecov-action@v5