Docker integration test for MacAddress setting

Over in Address CVE-2024-41110/GHSA-v23v-6jw2-98fq by u... (!4925 - merged) I attempt to fix https://gitlab.com/gitlab-org/gitlab-runner/-/issues/37919, but fully doing so requires subtle changes to setting the MacAddress property of build containers. This MR introduces a retrospective integration tests to fully capture the effect that configuring a docker executor with a MacAddress has on the resulting build container, so we can be sure to not break anytging while fixing https://gitlab.com/gitlab-org/gitlab-runner/-/issues/37919+

Run just this test with go test -v -count 1 -tags integration -run TestDockerCommand_MacAddressConfig ./executors/docker/...