Add readonlyRootfs flag to apps table

When turned off, it will put the app in a writable rootfs. This
allows us to debug live/production apps (like change start.sh) and
just get them up and running. Once turned off, this app cannot be
updated anymore (unless the force flag is set). This way we can
then update it using the CLI if we are convinced that the upcoming
update fixes the problem.

Part of #171

src/docker.js

@@ -197,7 +197,7 @@ function createSubcontainer(app, name, cmd, options, callback) {
                 MemorySwap: memoryLimit, // Memory + Swap
                 PortBindings: isAppContainer ? dockerPortBindings : { },
                 PublishAllPorts: false,
-                ReadonlyRootfs: !developmentMode, // see also Volumes in startContainer
+                ReadonlyRootfs: app.readonlyRootfs,
                 RestartPolicy: {
                     "Name": isAppContainer ? "always" : "no",
                     "MaximumRetryCount": 0