Add Docker resolver configuration package

Add configuration toml file format and configuration
function to configure registry hosts from a directory
based configuration. Compatible with Docker registry
certificate loading.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

remotes/docker/config/config_unix.go

@@ -0,0 +1,40 @@
+// +build !windows
+
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package config
+
+import (
+	"crypto/x509"
+	"path/filepath"
+)
+
+func hostPaths(root, host string) []string {
+	ch := hostDirectory(host)
+	if ch == host {
+		return []string{filepath.Join(root, host)}
+	}
+
+	return []string{
+		filepath.Join(root, ch),
+		filepath.Join(root, host),
+	}
+}
+
+func rootSystemPool() (*x509.CertPool, error) {
+	return x509.SystemCertPool()
+}