Attempt to make binderhub software allowed to read if an image is available in quay.io

[consideRatio]

Attempt to fix #4315, practically by using extraVolumes and extraVolumeMounts config introduced from 2i2c-org/binderhub-service#122, in order to mount the k8s Secret created by the binderhub-service chart when configuring buildPodsRegistryCredentials.

The name of the secret to mount should be named something like small-binder-binderhub-service-build-pods-docker-config, and you can configure the location it was mounted to be detected by binderhub via binderhub-service.config.DockerRegistry.docker_config_path.

Testing of this can be done in opensci's small binderhub, where its verified by building a repo twice and the second time it doesn't do a build at all but just launches, after success, we should update all hubs using quay.io in the same way.