Rebase the Dockerfile off of nubs/phpunit.

nubs/phpunit (and its parent nubs/composer-build) provide a much better
base for our build docker images.  They ensure that the build isn't run
as root, setup a more sane environment (with open_basedir restrictions,
correct environment variables, fully operational composer, etc.), and
handle abstracting out the core composer and phpunit dependencies.

This also moves the `set-env.sh` script into `/home/build`.  That
doesn't require root to create or execute, so it should belong to the
build user.