builddebs: Install man-db for builders

Some package builds pull in Java (such as trafficserver); Installation of Java on Debian container instance will fail due to an old upstream Debian bug [1]. To work around this, we can either manually create the man directory or install man-db. To prevent mucking around the system dirs let's just install man-db.

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=955619