Debian images: Use locale-gen instead of localdef

The use of manually calling localdef caused any future update to the
locales package to remove the manually installed locales, since
locale-gen takes precendence. This would usually be encountered when
a downstream Dockerfile added additional packages, and as a side effect
caused an upgrade to the locales package.

Fix by relying on the /etc/locale.gen file, which is the official place
to specify which locales should be installed.

Fixes #1112

Author: lfittl