feat: route based on hostname rather than path

[Addono]

Currently, the proxy infers the target hostname based on the path of the repository. However, this means that all calls which are not targeted directly to retrieving an image, e.g. /v2/ calls, do not include the target hostname and thus cannot be forwarded.

[Addono]

In future versions this might be picked up. To further document the working of such feature:

The proxy would support parsing its subdomain to infer the target registry:

docker pull hub.registry.docker.com.example.com/addono/container-registry-proxy

This would remove the need to configure the proxy with the desired repository, allowing different repositories to be queried from the same proxy instance. For now, this is regarded a more advanced use-case outside of the scope of the current efforts. However, we do see the value for such flexibility, it might very well be added in future versions.