moved from static DNS provider to a dynamic Docker variable and env file

(see https://docs.docker.com/compose/compose-file/#version-top-level-element)

docker-compose.yml

@@ -1,6 +1,4 @@
 ---
-version: "3"
-
 services:
   traefik:
     image: traefik:${TRAEFIK_VERSION}
@@ -13,7 +11,7 @@ services:
       - --api.dashboard=true
       # configure Let's Encrypt automatic certificates
       - --certificatesresolvers.${TRAEFIK_CERTRESOLVER}.acme.dnschallenge=true
-      - --certificatesresolvers.${TRAEFIK_CERTRESOLVER}.acme.dnschallenge.provider=hetzner
+      - --certificatesresolvers.${TRAEFIK_CERTRESOLVER}.acme.dnschallenge.provider=${TRAEFIK_DNSPROVIDER}
       - --certificatesresolvers.${TRAEFIK_CERTRESOLVER}.acme.email=${LETSENCRYPT_EMAIL}
       - --certificatesresolvers.${TRAEFIK_CERTRESOLVER}.acme.keytype=RSA4096
       - --certificatesresolvers.${TRAEFIK_CERTRESOLVER}.acme.storage=/certs.json
@@ -32,8 +30,8 @@ services:
       # should not need, but just in case, a folder for dynamic config files is also configured
       - --providers.file.directory=/config
       - --providers.file.watch=true
-    environment:
-      - HETZNER_API_KEY=${HETZNER_API_KEY}
+    env_file:
+      - ${TRAEFIK_DNSPROVIDER_ENVFILE}
     labels:
       # expose Træfik using Træfik (dashboard)
       - traefik.enable=${TRAEFIK_ENABLED}

env.dnsprovider.example

@@ -0,0 +1 @@
+HETZNER_API_KEY=

env.example

@@ -11,7 +11,8 @@ TRAEFIK_PILOT_TOKEN=
 TRAEFIK_TLSENABLED=true
 
 # Certificate provider
-HETZNER_API_KEY=
+TRAEFIK_DNSPROVIDER=hetzner
+TRAEFIK_DNSPROVIDER_ENVFILE=./.env.dnsprovider
 LETSENCRYPT_EMAIL=admin@mydomain.com
 
 # Debugging