docker: rebase again on hpk/tls-external and modify our end

Remove the custom certmon timer (polling via sha1sum every 60s),
replaced by the deployer's tls-cert-reload.path unit (inotify).

chatmail-init.sh: inject TLS_EXTERNAL_CERT_AND_KEY env var into
chatmail.ini at startup if not already present.

docker-compose.yaml: remove CMDEPLOY_STAGES, CHATMAIL_NOSYSCTL,
TLS_EXTERNAL_CERT_AND_KEY from base environment (set in init script
or via override file).

docker-compose.yaml

@@ -36,9 +36,6 @@ services:
         max-file: "3"
     environment:
       MAIL_DOMAIN: $MAIL_DOMAIN
-      CMDEPLOY_STAGES: ${CMDEPLOY_STAGES:-}
-      CHATMAIL_NOSYSCTL: ${CHATMAIL_NOSYSCTL:-True}
-      TLS_EXTERNAL_CERT_AND_KEY: ${TLS_EXTERNAL_CERT_AND_KEY:-}
     network_mode: "host"
     volumes:
       ## system (required)