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.

See also #1246 for discussion.
Any reason we can't move this up into the main install block? I'd prefer to avoid two updates if we can help it. We can add the Docker repository, then do an update and install all packages in one fell swoop.