Fix docker compose down removing images still in use

[Pnkcaht]

What I did

Adjusted the image pruning logic used by docker compose down to avoid removing images that are still in use by existing containers.

Before this change, Compose could attempt to remove images that were still referenced by running or stopped containers belonging to the same project, which resulted in unexpected behavior and errors during down --rmi.

The pruner now checks containers associated with the project and filters out images that are currently in use before scheduling them for removal.

Related issue

Fixes #13491

(not mandatory) A picture of a cute animal, if possible in relation to what you did

🐳 A whale carefully carrying containers instead of dropping them — just like Compose now avoids removing images that are still in use.

Notes

• Only affects docker compose down with image removal enabled
• Prevents removal of images still referenced by project containers
• Existing behavior preserved for unused images
• Covered and validated by unit tests (TestDownRemoveImages, TestDownRemoveImages_NoLabel)

[ndeloof]

engine detects image are in use by another container, simpler (imho) to let it manage this check (which will then not suffer from potential race conditions) and manage error during image removal attempts

[Pnkcaht]

What was changed

@ndeloof
The image pruning logic used by docker compose down --rmi was updated to rely
on the Docker engine to detect whether images are currently in use by containers.

The pre-check that attempted to determine image usage inside Compose was
removed. Images are now passed directly to the engine for removal, and any
errors returned by the engine are propagated.

A code comment was restored to document that image usage checks are intentionally
handled by the engine during removal.

Tests

Manual end-to-end testing was performed using a minimal Compose project with a
single service using the nginx:alpine image.

Test flow:

1. Start the service using docker compose up -d
2. Run docker compose down --rmi all
3. Confirm that:
   • The container is removed
   • The image is removed when no container references it
4. Run docker compose up -d again and confirm the image is pulled again
5. Stop the service and run docker compose down --rmi all once more to confirm
   the image is removed again

Terminal output screenshots are attached below to show the observed behavior.

Image 1

Image 2 (Continuation)

[ndeloof]

still many unrelated changes and removed comments without reason.
I'm not against use of AI to bake a pull-request and make better code in a shorter time, but please review the result before submitting.
Closing this one as we all waste our time