Creating soft links to files that do not exist is just madness, let's
stop it. Instead, we take advantage of the CONFIG_ROOT environment
variable, which allows to place lms/cms.env/auth.json files anywhere.
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
