ContainerMetadataFetcher currently allows three hosts for container metadata endpoints: 169.254.170.2, localhost, and 127.0.0.1. However, neither localhost or 127.0.0.1 represent the local docker host. Instead, the host's localhost is reachable as host.docker.internal (docs). On Linux, the workaround is to use --network host, but on Mac and Windows this doesn't work (the docker network is always isolated from the host).

host.docker.internal should be added to ContainerMetadataFetcher's list of approved hosts, so that a custom metadata endpoint for testing can be run on the host rather than needing to be run in another container.