Install language-pack-en-base to ensure en_US.UTF-8 locale is present

At least the python Heroku buildpack sets en_US.UTF-8 as the default
locale if none is present. Dokku does not set a default locale, and
without the en_US.UTF-8 locale present this results in an ASCII locale
during the buildstep. Some python packages have UTF-8 encoding in their
source files which causes the buildstep to bomb out with a decoding
error.