Fix docker-compose not forwarding SECRET_KEY into container

SECRET_KEY, APP_URL, and DEBUG were hardcoded/missing in the compose
environment block, so passing SECRET_KEY from the host env had no effect
and the container always raised ImproperlyConfigured in production mode.

All three are now forwarded via ${VAR} substitution, consistent with
the other configurable values.

docker-compose.yml

@@ -1,16 +1,17 @@
 ---
 services:
   timetracker:
-    image: ${REGISTRY_URL:-registry.kucharczyk.xyz}/timetracker:1.7.0
+    image: ${REGISTRY_URL:-registry.kucharczyk.xyz}/timetracker:latest
     build:
       context: .
       dockerfile: Dockerfile
     container_name: timetracker
     environment:
-      - DEBUG=false
+      - DEBUG=${DEBUG:-false}
+      - SECRET_KEY=${SECRET_KEY}
       - TZ=${TZ:-Europe/Prague}
       # APP_URL drives ALLOWED_HOSTS and CSRF_TRUSTED_ORIGINS unless overridden.
-      - APP_URL=https://tracker.kucharczyk.xyz
+      - APP_URL=${APP_URL:-https://tracker.kucharczyk.xyz}
       - PUID=${PUID:-1000}
       - PGID=${PGID:-100}
       - DATA_DIR=${DATA_DIR:-/home/timetracker/app/data}