The `32bit/ubuntu:14.04` image seems to have been pulled by the owner. A
very recent commit to their repository recommends switching to
`ioft/i386-ubuntu:14.04` as the new 'official' image.
apt-get will attempt to use the dialog frontend by default.
When building with docker, error messages get written by debconf when
installing some packages because it tries to open the dialog frontend.
Set `DEBIAN_FRONTEND=noninteractive` to suppress these unnecessary
messages. The dialog frontend shouldn't be getting used anyway.