Add nginx behind traefik in docker-compose + image updates

Add nginx behind traefik in docker-compose + image updates

- support/docker/production/docker-compose.yml: addition of a nginx
image reusing support/nginx/peertube nginx conf to improve performance,
and lessen setup differences between the docker-compose install and the
typical production install.
- support/docker/production/docker-compose.yml: postgres 10 -> postgres
12, redis 4 -> redis 5. Postgres major updates implies manual upgrade.
- support/nginx/peertube: HTTP -> HTTPS redirection is now commented
by default, to allow its reuse in support/docker/production/docker-compose.yml.