Allow registry clients to connect via http2

Http2 will be enabled by default and can be disabled with a configuration option.

Signed-off-by: Adam Duke <adam.v.duke@gmail.com>

registry/registry.go

@@ -116,7 +116,7 @@ func (registry *Registry) ListenAndServe() error {
 	if config.HTTP.TLS.Certificate != "" || config.HTTP.TLS.LetsEncrypt.CacheFile != "" {
 		tlsConf := &tls.Config{
 			ClientAuth:               tls.NoClientCert,
-			NextProtos:               []string{"http/1.1"},
+			NextProtos:               nextProtos(config),
 			MinVersion:               tls.VersionTLS10,
 			PreferServerCipherSuites: true,
 			CipherSuites: []uint16{
@@ -343,3 +343,12 @@ func resolveConfiguration(args []string) (*configuration.Configuration, error) {
 
 	return config, nil
 }
+
+func nextProtos(config *configuration.Configuration) []string {
+	switch config.HTTP.HTTP2.Disabled {
+	case true:
+		return []string{"http/1.1"}
+	default:
+		return []string{"h2", "http/1.1"}
+	}
+}