Add support for registry host path override

Adds support for mirrors which are non-compliant with the
OCI distribution specification but have previously mirrored
content with a namespace prefix after the API root `/v2`.

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

docs/hosts.md

@@ -274,6 +274,17 @@ or
   x-custom-1-2 = "another custom header"
 ```
 
+## override_path field
+
+`override_path` is used to indicate the host's API root endpoint is defined
+in the URL path rather than by the API specification. This may be used with
+non-compliant OCI registries which are missing the `/v2` prefix.
+(Defaults to `false`)
+
+```
+override_path = true
+```
+
 ## host field(s) (in the toml table format)
 
 `[host]."https://namespace"` and `[host].http://namespace` entries in the
@@ -310,6 +321,10 @@ for this registry host namespace:
 
 [host."https://test-3.registry"]
   client = ["/etc/certs/client-1.pem", "/etc/certs/client-2.pem"]
+
+[host."https://non-compliant-mirror.registry/v2/upstream"]
+  capabilities = ["pull"]
+  override_path = true
 ```
 
 **Note**: Recursion is not supported in the specification of host mirror

remotes/docker/config/hosts.go

@@ -291,7 +291,7 @@ type hostFileConfig struct {
 	// Accepted types
 	// - string - Single file with public and private keys
 	// - []string - Multiple files with public and private keys
-	// - [][2]string - Muliple keypairs with public and private keys in separate files
+	// - [][2]string - Multiple keypairs with public and private keys in separate files
 	Client interface{} `toml:"client"`
 
 	// SkipVerify skips verification of the server's certificate chain
@@ -302,6 +302,12 @@ type hostFileConfig struct {
 	// Header are additional header files to send to the server
 	Header map[string]interface{} `toml:"header"`
 
+	// OverridePath indicates the API root endpoint is defined in the URL
+	// path rather than by the API specification.
+	// This may be used with non-compliant OCI registries to override the
+	// API root endpoint.
+	OverridePath bool `toml:"override_path"`
+
 	// TODO: Credentials: helper? name? username? alternate domain? token?
 }
 
@@ -374,16 +380,12 @@ func parseHostConfig(server string, baseDir string, config hostFileConfig) (host
 		}
 		result.scheme = u.Scheme
 		result.host = u.Host
-		// TODO: Handle path based on registry protocol
-		// Define a registry protocol type
-		//   OCI v1    - Always use given path as is
-		//   Docker v2 - Always ensure ends with /v2/
 		if len(u.Path) > 0 {
 			u.Path = path.Clean(u.Path)
-			if !strings.HasSuffix(u.Path, "/v2") {
+			if !strings.HasSuffix(u.Path, "/v2") && !config.OverridePath {
 				u.Path = u.Path + "/v2"
 			}
-		} else {
+		} else if !config.OverridePath {
 			u.Path = "/v2"
 		}
 		result.path = u.Path

remotes/docker/config/hosts_test.go

@@ -104,6 +104,13 @@ ca = "/etc/path/default"
 
 [host."https://test-3.registry"]
   client = ["/etc/certs/client-1.pem", "/etc/certs/client-2.pem"]
+
+[host."https://noncompliantmirror.registry/v2/namespaceprefix"]
+  capabilities = ["pull"]
+  override_path = true
+
+[host."https://noprefixnoncompliant.registry"]
+  override_path = true
 `
 	var tb, fb = true, false
 	expected := []hostConfig{
@@ -159,6 +166,17 @@ ca = "/etc/path/default"
 				{filepath.FromSlash("/etc/certs/client-2.pem")},
 			},
 		},
+		{
+			scheme:       "https",
+			host:         "noncompliantmirror.registry",
+			path:         "/v2/namespaceprefix",
+			capabilities: docker.HostCapabilityPull,
+		},
+		{
+			scheme:       "https",
+			host:         "noprefixnoncompliant.registry",
+			capabilities: allCaps,
+		},
 		{
 			scheme:       "https",
 			host:         "test-default.registry",