docker: add Traefik support

USE_FOREIGN_CERT_MANAGER existed in compose/example.env but was never
read by any code. This wires it up end-to-end based on PR 662.

- Preliminarily add config options for this, and skip AcmetoolDeployer if
set.
- Add Traefik integration in docker/docker-compose-traefik.yaml, with
  traefik-certs-dumper
- post-hook.sh creates fullchain/privkey symlinks for chatmail
- Chatmail container uses ports 25/143/465/587/993 directly, Traefik
  handles 80/443
- docker/traefik/ contains config.yaml and dynamic configs
- docker/example-traefik.env for the Traefik setup
- rename USE_FOREIGN_CERT_MANAGER to CHATMAIL_NOACME

docker-compose.yaml

@@ -29,7 +29,7 @@ services:
     environment:
       MAIL_DOMAIN: $MAIL_DOMAIN
       CMDEPLOY_STAGES: ${CMDEPLOY_STAGES:-}
-      USE_FOREIGN_CERT_MANAGER: ${USE_FOREIGN_CERT_MANAGER:-}
+      CHATMAIL_NOACME: ${CHATMAIL_NOACME:-}
     network_mode: "host"
     volumes:
       ## system