libnetwork: ServeDNS(): don't panic on unsupported query types

This was added in b3c883bb2f, but resulted
in a panic if the embedded DNS had to handle an unsupported query-type,
such as ANY.

This patch adds a debug log for this case (to better describe how it's
handled.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

libnetwork/resolver.go

@@ -384,7 +384,8 @@ func (r *resolver) ServeDNS(w dns.ResponseWriter, query *dns.Msg) {
 	case dns.TypeSRV:
 		resp, err = r.handleSRVQuery(name, query)
 	default:
-		panic("error")
+		queryType := dns.TypeToString[query.Question[0].Qtype]
+		logrus.Debugf("[resolver] query type %s is not supported by the embedded DNS and will be forwarded to external DNS", queryType)
 	}
 
 	if err != nil {