docs: update instructions for basefile creation to include capabilities

When creating the basefile via tar without `--xattrs --xattrs-include='*.*'`,
the resulting basefile doesn't provide capabilities as used and needed
for example by ping(8), also see https://bugs.debian.org/881829

While at it, also exclude /var/lib/apt/lists/ files, to further reduce
the resulting basefile.

Closes: https://github.com/grml/grml-live/issues/143