Detect workdir if not specified in the manifest

Is your feature request related to a problem? Please describe. If the workdir is not specified in the manifest, we should try to detect it instead of always going to /usr/src/app, since it might not be available in the container.

Additional context we could get it from the SSH check we already do.

Another option, suggested by @pchico83

if sync is specified, it takes preference. if there is more than one, could we open more than one folder?
if sync is not specified, workdir is mandatory and it is what should be opened.