fix(Dockerfile): ensure `apt-get install` is prefixed by `update`

Since Dockerfile layers are cached, having `apt-get update` as a separate
command creates the potential for a subsequent `apt-get install` to fail.
Best practices (other than "install from source") seem to be to make
both commands into a one-liner.

This PR ensures we don't `apt-get install` as its own command anywhere,
removes the installation of some packages that were already in deis/base,
and removes a redundant ENV DEBIAN_FRONTEND statement.
Merge pull request #1249 from deis/dockerfile-apt-get-fix

fix(Dockerfile): ensure `apt-get install` is prefixed by `update`