docker: use the system dns for app containers

take 2 after failed attempt with 92bce26e22

this makes the dashboard domain resolve internally to nginx

can test with `getent ahosts my.domain.com` inside the container.

src/docker.js

@@ -373,10 +373,7 @@ async function createSubcontainer(app, name, cmd, options) {
     if (isAppContainer) {
         containerOptions.Hostname = app.id;
         containerOptions.HostConfig.NetworkMode = 'cloudron'; // user defined bridge network
-        // use unbound for app DNS resolution. this is a workaround for the requirement that OIDC server name has to resolve to
-        // public IP somehow skipping any /etc/hosts entries
-        containerOptions.HostConfig.Dns = ['172.18.0.1'];
-        containerOptions.HostConfig.DnsSearch = ['.'];
+        containerOptions.HostConfig.ExtraHosts = [ `${dashboardFqdn}:172.18.0.1` ];
 
         containerOptions.NetworkingConfig = {
             EndpointsConfig: {