Enable Authelia template config filter

configuration.yml uses Go-template substitution like
`{{ env "DOMAIN" }}` for the cookie domain, authelia_url, etc.
That substitution doesn't run by default; Authelia reads the
braces literally, fails URL parsing with `invalid character "{"
in host name`, and `domain` validation rejects the hostname for
having no period.

Setting X_AUTHELIA_CONFIG_FILTERS=template enables Authelia's
Go-template filter so the substitution runs at config load.
The env var is inherited by `docker compose run --rm authelia`,
so validate-config picks it up too.

https://claude.ai/code/session_013XZ1vmgk78k2PEQ5DmJhF3

docker-compose.yml

@@ -34,8 +34,14 @@ services:
       - AUTHELIA_SESSION_SECRET_FILE=/secrets/SESSION_SECRET
       - AUTHELIA_STORAGE_ENCRYPTION_KEY_FILE=/secrets/STORAGE_ENCRYPTION_KEY
       - TZ=${TZ:-UTC}
-      # Passed through so authelia/configuration.yml can use {{ env "DOMAIN" }}
-      # instead of hardcoding your domain in the config file.
+      # Enables Go-template substitution in configuration.yml so
+      # `{{ env "DOMAIN" }}` actually expands instead of being read as a
+      # literal string. Without this, Authelia parses the braces as part
+      # of the hostname and validate-config fails with `invalid character
+      # "{" in host name`. Inherited by `docker compose run --rm authelia`,
+      # so validate-config picks it up too.
+      - X_AUTHELIA_CONFIG_FILTERS=template
+      # Passed through so authelia/configuration.yml can use {{ env "DOMAIN" }}.
       - DOMAIN=${DOMAIN}
     volumes:
       - ./authelia:/config