[BUG] Images in use are not detected when using Swarm

[sergi0g]

Describe the bug
When an image is deployed with Docker Swarm, Cup does not detect it as in use.

To Reproduce

1. Deploy Cup and any other image (or just Cup itself) using Docker Swarm on a server. Make sure those images are only used in Swarm.
2. Open the Cup dashboard and use the filter to hide unused images.
3. Notice that any images deployed exclusively with Swarm are hidden. This is also reflected in the in_use JSON field in the API response.

Expected behavior
Any Swarm images in use should not be hidden.

System info:
Occurs on all systems.

Additional context
The logic that detects images in use is probably not very well written and completely ignores the existence of Swarm. More care should be taken during testing.