"env" now only generates the environment, and depends only on
config.json, which is run only when necessary. There exists only one
"make configure" command, which force-runs config.json and builds the
env.
By switching from ubuntu:18.04 to alpine, we reduce the local image size
from 510 to 87 Mb. Because we don't use bash anymore, it's difficult to
rely on chroot, so we just get rid of docker-entrypoint.sh