Dockerfile: clean up

This changes the Dockerfile to make the following improvements:
- switch to the unprefixed ubuntu image
- stop adding the universe repo
it's already in the latest stackbrew/ubuntu images by default
- merge the apt-get update and install operations
- remove the package lists in the same apt-get update & install step

These changes reduce the image size reported by `docker images` from 432 MB to 399 MB and gets rid of one layer.