allowing BAN_template to be populated from docker env

docker-compose.yml

@@ -15,3 +15,4 @@ services:
       MAX_DAYS: "${MAX_DAYS}"            # strings are converted to ints when the
       MAX_POSTS: "${MAX_POSTS}"          # corresponding config value is an int
       SLEEP_MINUTES: "${SLEEP_MINUTES}"  
+      BAN_TEMPLATE: "${BAN_TEMPLATE}"

populate_config.py

@@ -11,6 +11,7 @@ CONFIG_KEYS = [
     "max_days",
     "max_posts",
     "sleep_minutes",
+    "ban_template",
 ]
 
 DEFAULTS = {
@@ -23,6 +24,7 @@ DEFAULTS = {
     "max_days": 180,
     "max_posts": 180,
     "sleep_minutes": 5,
+    "ban_template": "",
 }
 
 # Try both plain and DP_ prefix for env vars