Set LANG on all Linux variants again, including 3.13+ (#1145)

Restore `ENV LANG C.UTF-8` on 3.13 and later, undoing the 3.13+ part of 0d53911.  Without it, two groups of users break: embedded interpreters (uWSGI, mod_wsgi) before 3.15, which fall back to ASCII filesystem and stdio encodings because they don't get the locale coercion and UTF-8 mode that the `python` CLI does, and alpine on every version through 3.16-rc, where musl never gets `LC_CTYPE` set so libraries that read locale environment variables (babel) find nothing.  This is the same class of failure that got the original removal reverted in f871d04.

Replace the template comment with those concrete reasons, and add a one-line `docker run` reproducer for the embedded case.  Regenerate the Dockerfiles.

Co-authored-by: Tianon Gravi <tianon.gravi@docker.com>

Author: tianon