use dir mount instead of file mount

file mounting is fraught with problems wrt change notifications.

first, we must be carefule that the inode does not change.

second, changes outside container do not result in fs events inside the container.
haraka cache settings files and relies on fs events. So, even
though the file gets updated inside the container, haraka doesn't
see it.

https://github.com/docker/docker/issues/15793

src/infra_version.js

@@ -17,7 +17,7 @@ exports = module.exports = {
         'postgresql': { repo: 'cloudron/postgresql', tag: 'cloudron/postgresql:0.15.0' },
         'mongodb': { repo: 'cloudron/mongodb', tag: 'cloudron/mongodb:0.11.0' },
         'redis': { repo: 'cloudron/redis', tag: 'cloudron/redis:0.10.0' },
-        'mail': { repo: 'cloudron/mail', tag: 'cloudron/mail:0.26.0' },
+        'mail': { repo: 'cloudron/mail', tag: 'cloudron/mail:0.27.0' },
         'graphite': { repo: 'cloudron/graphite', tag: 'cloudron/graphite:0.10.0' }
     }
 };