Comment 4 for bug 1860388

www-data has id 33, so belongs to the first class described at https://www.debian.org/doc/debian-policy/ch-opersys.html#uid-and-gid-classes

It's therefore reasonable for packaging to assume that it always exists, so this is not a bug in nginx packaging. In the general case it's not reasonable for packaging to accommodate arbitrary situations that cannot arise by policy, since that would make packaging complexity explode. I don't think this case warrants an exception.

I believe that the root cause of your problem is your hacked image. Removing the www-data user or group can be expected to have exactly these consequences.