New Feature Request: support multi fallbacks

Multiple fallbacks are supported.

One implementation for reference:

• serverless-registry can be configured with multiple domain names. For example: docker-mirror.example.com, quay-mirror.example.com
• Config Multiple fallbacks
• Depending on the requested domain name, fallback to the corresponding backend. For example, if the requested domain name is: docker-mirror.example.com, then fallback to docker.io.