Add namespace query parameter for registry proxying

Proxy registries are designed to serve content from upstreams.
However, the proxy hostname will usually not match the hostname
of the upstream, requiring the proxy to only use a single
upstream or use its own pattern matching to determine the upstream.
To solve this issue, the client will pass along the namespace which
is being used for the request, allowing mirrors to easily map
to multiple upstreams. This query parameter can safely be ignored
if multiple upstreams are not supported.

Signed-off-by: Derek McGowan <derek@mcg.dev>

remotes/docker/registry.go

@@ -73,6 +73,15 @@ type RegistryHost struct {
 	Header       http.Header
 }
 
+func (h RegistryHost) isProxy(refhost string) bool {
+	if refhost != h.Host {
+		if refhost != "docker.io" || h.Host != "registry-1.docker.io" {
+			return true
+		}
+	}
+	return false
+}
+
 // RegistryHosts fetches the registry hosts for a given namespace,
 // provided by the host component of an distribution image reference.
 type RegistryHosts func(string) ([]RegistryHost, error)