Run a command before live_update sync

[daveagill]

Describe the Feature You Want

We want the ability to run a command before the live_update sync() calls in order to stop a wrapped child process before syncing files.

Current Behavior

According to the documentation run() must come after sync(). So syncing happens while the process is running.

Why Do You Want This?

Syncing in files while the process is running can cause instability and/or the application could hold file locks. We would like to cleanly stop the (wrapped/child) process -> then sync the files -> then launch the new process. This would be easily done if it were possible to place a run() command before sync() or if docker_build supported a pre_live_update option.