nginx:stable-alpine

use nginx stable-alpine image instead of debian

jwilder/nginx-proxy allows Per-VIRTUAL_HOST settings

- https://github.com/jwilder/nginx-proxy#per-virtual_host

which we used to set `client_max_body_size 200M;` for https://base.transformap.co.
Early warnings due to a missing /etc/nginx/vhost.d/default can be omitted.

* https://github.com/jwilder/nginx-proxy/issues/304#issuecomment-189775983

what worked before the refactoring