This is more canonical with stuff 'elsewhere on the internet'
we already did it behind-the-scenes for 'recommended gunicorn', so the
harmonization is in having the same default for the rest.
* I'd rather harmonize with "the docker way" than with my own recommended setup
* slim: expectation is that this is smaller (not checked)
* this is a 2-stage build (slim has no gcc, needed for mysql)
* PEP668 does not apply here ("externally managed"), which leads to
* better ergonomics: no env/bin/some-command