Using the dependency proxy for Docker images automatically
The Docker part of the dependency proxy is for Docker Hub images only, e.g.
Because the PAAS runner actually receives the full job description and then dispatches it to the provisioned resources, we have the opportunity to inspect all
services values, to prepend the dependency proxy URL prefix on the fly if they point to Docker Hub.
This is simpler for users, and provides safety for the PAAS service by avoiding its outgoing IP addresses to be banned from Docker Hub because of the rate limits (could otherwise happen if users didn't bother using the dependency proxy).