Always clean up intermediate multiplatform images on exit

[shanejbrown]

Summary

Intermediate per-platform Docker images created during multiplatform builds were left behind in the local Docker daemon after buildrunner finished. This change ensures they are always removed when the MultiplatformImageBuilder context manager exits.

Changes

• Add _cleanup_intermediate_images() method to MultiplatformImageBuilder that iterates over all built platform images and force-removes them from the local Docker daemon
• Call the cleanup in __exit__ before stopping the local registry, so images are removed regardless of success or failure
• Bump version from 3.24 to 3.25

Merge requirements satisfied?

• I have updated the documentation or no documentation changes are required.
• I have added tests to cover my changes.
• I have updated the base version in pyproject.toml (if appropriate).