Pin mher/flower Docker image to fix startup error

fastapi#398
fastapi#399

Error:

```text
ERROR: for flower  Cannot start service flower: OCI runtime create
failed: container_linux.go:380: starting container process caused:
exec: "celery --broker=amqp://guest@queue:5672//":
stat celery --broker=amqp://guest@queue:5672//:
no such file or directory: unknown
```

Author: br3ndonland

Diff for: {{cookiecutter.project_slug}}/docker-compose.yml

@@ -111,7 +111,7 @@ services:
     # You also have to change the flower command
 
   flower:
-    image: mher/flower
+    image: mher/flower:0.9.7
     networks:
       - ${TRAEFIK_PUBLIC_NETWORK?Variable not set}
       - default