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/traefik/post-hook.sh

@@ -0,0 +1,12 @@
+#!/bin/sh
+# Post-hook for traefik-certs-dumper: create symlinks from Traefik's
+# cert dump format to the paths chatmail expects (fullchain, privkey).
+CERTS_DIR="${CERTS_DIR:-/certs}"
+
+for dir in "$CERTS_DIR"/*/; do
+    [ -d "$dir" ] || continue
+    cd "$dir"
+    [ -f "certificate.crt" ] && ln -sf certificate.crt fullchain
+    [ -f "privatekey.key" ] && ln -sf privatekey.key privkey
+    cd - > /dev/null
+done