Tarballs generation for the Rails app
It is also probably useful for source installations: don't need to install an arbitrary version of Mercurial before the exact specification is available and various downstream packaging efforts.
Of course we'd like to do this in a CI job, using
hg archive and running automatically for Heptapod tags.
Because we want to be able to push intermediate testing Docker images, we also need to produce tarballs for arbitrary revisions, perhaps with a manual job.