Document about OS-packaged modules vs docker-perl

[zakame]

Debian Perl packages are not guaranteed to work (much more even detected) against the Perl we build here, especially as these perls might not even have the same version and @INC paths...

Let me prepare a PR on docker-library/docs for this, also the salient points are in #26 (comment)

Originally posted by @zakame in #92 (comment)