Fix docker image to not log at `/homeserver.log` (#10045)

Fixes #9970

Signed-off-by: Sergio Miguéns Iglesias <lonyelon@lony.xyz>

changelog.d/10045.docker

+Fix bug introduced in Synapse 1.33.0 which caused a `Permission denied: '/homeserver.log'` error when starting Synapse with the generated log configuration. Contributed by Sergio Miguéns Iglesias.

docker/conf/log.config

@@ -9,10 +9,11 @@ formatters:
 {% endif %}
 
 handlers:
+{% if LOG_FILE_PATH %}
   file:
     class: logging.handlers.TimedRotatingFileHandler
     formatter: precise
-    filename: {{ LOG_FILE_PATH or "homeserver.log" }}
+    filename: {{ LOG_FILE_PATH }}
     when: "midnight"
     backupCount: 6  # Does not include the current log file.
     encoding: utf8
@@ -29,6 +30,7 @@ handlers:
     # be written to disk.
     capacity: 10
     flushLevel: 30  # Flush for WARNING logs as well
+{% endif %}
 
   console:
     class: logging.StreamHandler