ArgumentError: directory for pid=/.../unicorn.pid not writable

Looks like the conclusion here is that we need to update the minimum required Docker version check in

Is there a place other than this forum where this should be reported as a bug?

It’s not really a bug. It’s simply not enforcing a minimum version for docker. Enforcing minimum versions is fraught because enforcing a current version too soon is also something that is annoying.

The person who is best qualified to update the code is aware of the issue. It’s likely just not a high priority since if you keep your software up to date, you’d not have had an issue.