Comment 2 for bug 1388742

man(5) shows the syntax for the setuid stanza as:

setuid USERNAME

So, clearly 'setuid 1000' is not valid and hence neither is your job. Attempting to start a job manually which specifies 'setuid 1000' fails with the error:

start: Job failed to start

I have so far been unable to recreate your hang. Please specify which versions of Ubuntu are being used inside and outside the container.