Set DNS per container rather than the daemon

All Cloudron containers need to have the nameserver 172.18.0.1. This was
being done at the daemon level, however since there are also iptables
rules restricting access to the nameserver from containers that aren't
on the Cloudron Docker network, this broke DNS for non-Cloudron
containers.

Since the DNS is only required for Cloudron containers in the first
place, this patch specifies 172.18.0.1 as the nameserver when Cloudron
creates a container and reverts the change at the daemon level

src/docker.js

@@ -203,6 +203,7 @@ function createSubcontainer(app, name, cmd, options, callback) {
                 CpuShares: 512, // relative to 1024 for system processes
                 VolumesFrom: isAppContainer ? null : [ app.containerId + ":rw" ],
                 NetworkMode: isAppContainer ? 'cloudron' : ('container:' + app.containerId), // share network namespace with parent
+                Dns: ['172.18.0.1'], // use internal dns
                 SecurityOpt: enableSecurityOpt ? [ "apparmor:docker-cloudron-app" ] : null // profile available only on cloudron
             }
         };