Bump fireflyiii/core from version-6.5.4 to version-6.5.6 in /docker (#51)

Bumps fireflyiii/core from version-6.5.4 to version-6.5.6.

---
updated-dependencies:
- dependency-name: fireflyiii/core
  dependency-version: version-6.5.6
  dependency-type: direct:production
...

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Author: dependabot[bot]

docker/docker-compose.yml

@@ -328,7 +328,7 @@ services:
       retries: 5
 
   firefly_app:
-    image: fireflyiii/core:version-6.5.4
+    image: fireflyiii/core:version-6.5.6
     container_name: firefly_app
     hostname: firefly_app
     restart: unless-stopped