Set container hostname to something constant

currently, it defaults to the container name. apps might use this hostname
for communication with child containers. but this breaks if app gets updated
and gets a new hostname

src/docker.js

@@ -218,6 +218,7 @@ function createSubcontainer(app, name, cmd, options, callback) {
         var containerOptions = {
             name: name, // used for filtering logs
             Tty: isAppContainer,
+            Hostname: app.id, // set to something 'constant' so app containers can use this to communicate (across app updates)
             Image: app.manifest.dockerImage,
             Cmd: (isAppContainer && app.debugMode && app.debugMode.cmd) ? app.debugMode.cmd : cmd,
             Env: stdEnv.concat(addonEnv).concat(portEnv).concat(appEnv),