The "latest" tag is a pain to maintain: it's a tag that we delete and
re-create at every release. Whenever we delete it, the binaries become
unavailable on Github until they are re-generated. Thus, from now on, we
conform to good practices (as examplified by the
github.com/docker/compose) project and distribute only pinned release.
The "nightly" tag remains, for now, as it allows us to distribute beta
features. It may disappear in the future.
The TUTOR_USER env variable is not available anymore after running "su",
which causes the tutor home to be in /home/.local/share/tutor. So we
just get rid of this variable entirely.
Environment is no longer generated separately for each target, but only
once the configuration is saved.
Note that the environment is automatically updated during
re-configuration, based on a "version" file stored in the environment.
Pulling images is slow, often unnecessary, and sometimes suprising for
the user, as the newer images may break the older tutor version.
Instead, we add a "--pullimages" option to "quickstart" to explicitely
update docker images.
