admin: Disable host checking if wildcard interface is specified

To clarify, listening on wildcard interfaces is NOT the default and
should only be done under certain circumstances and when you know
what you're doing. Emits a warning in the log.

Fixes https://github.com/caddyserver/caddy-docker/issues/71

listeners.go

@@ -302,6 +302,16 @@ func (na NetworkAddress) isLoopback() bool {
 	return false
 }
 
+func (na NetworkAddress) isWildcardInterface() bool {
+	if na.Host == "" {
+		return true
+	}
+	if ip := net.ParseIP(na.Host); ip != nil {
+		return ip.IsUnspecified()
+	}
+	return false
+}
+
 func (na NetworkAddress) port() string {
 	if na.StartPort == na.EndPort {
 		return strconv.FormatUint(uint64(na.StartPort), 10)