[Dockerfiles] Do not persist DEBIAN_FRONTEND=noninteractive in images

According to https://github.com/moby/moby/issues/4032#issuecomment-163689851
(and some other comments in the issue) it's not recommended to set
`DEBIAN_FRONTEND` via `ENV` in a Dockerfile.  `ARG` has the same
effect at build time but does not change `DEBIAN_FRONTEND` in the
final image, so I switched to it.  It should also work to remove it
completely.