Remove reference to submitdir in domserver Dockerfile

[tom93]

It was removed in DOMjudge/domjudge@d66725038, causing the chown command to fail.

This change will cause issues if the new Dockerfile is used with an older release that still has a submissions directory: the directory will no longer be readable/writable by the www-data user. I'm not sure if that's a problem.

[nickygerritsen]

Probably it’s better to add a if [[ -d bla ]] around this line so we can still build valid images for older versions.

[nickygerritsen]

Thanks!