Add Test for port allocation bug (port already in use by other programs than docker)

Signed-off-by: Tibor Vass <teabee89@gmail.com>
Fix an issue where already allocated ports would not trigger an error.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
Fix my own comments from #7927

Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com>
Merge pull request #7934 from LK4D4/fix_double_allocation

Fix error propagation from userland-proxy