WaitForServicesTimeout no longer supports -1 to disable timeout

In Gitlab Runner 18.7.0, we're seeing jobs fail with ERROR: Preparation failed: getting exposed ports: service failed to start after 109ns

It looks like this code e69da034 doesn't handle the case where e.Config.Docker.WaitForServicesTimeout is -1, which was recommended by ttps://gitlab.com/gitlab-org/gitlab-runner/-/blob/main/docs/configuration/advanced-configuration.md?ref_type=heads#the-runnersdocker-section