Use '-s empty' instead of '-s dir'

This *should* have the same effect as the previous strategy: Instead of
'mkdir empty; fpm -s dir -C empty ...' we can simply do 'fpm -s empty'

Docker-DCO-1.1-Signed-off-by: Jordan Sissel <jls@semicomplete.com> (github: jordansissel)
Merge pull request #3555 from jordansissel/patch-1

Update Ubuntu packaging script to use 'fpm -s empty' instead of 'mkdir empty && fpm -s dir -C empty'