Use isolated docker config in test sandbox

[le-lenn]

The test sandbox was mounting ${HOME}/.docker/config.json into the manager/worker containers. That made the tests depend on my local Docker config, which was not compatible with the test suite.

This change sets DOCKER_CONFIG=/tmp/docker-config and mounts an empty named volume, keeping the test environment hermetic and consistent across machines.

[m90]

The idea behind the mount is to achieve the opposite of an isolated config: in CI the host's credentials need to propagate into the container (which is running Docker in Docker) so that pulls are authenticated and builds don't get rate limited (which happens when Dependabot opens multiple PRs at once). Merging this PR would break that behavior.

If you wanted to make this more versatile, it would still need to support the option of mounting the config file as it's done right now.

[le-lenn]

This PR now adds the option to overwrite the docker config. I am not 100% happy with the approach, but I think its a good start for a discussion