Use generated images in integration test

[stepchowfun]

Replace the Alpine-based integration-test images with locally generated scratch images that each contain a unique 100 MiB payload. This avoids implicit pulls and makes the eviction threshold derive from Docker’s own image accounting instead of a fixed guess.

Status: Ready

Fixes: N/A

Details:

• Builds the protected and evictable images before Docuum starts, then computes the threshold from docker system df plus a 200 MB margin.
• Builds the newest image after the event subscription is live so it crosses the computed threshold and evicts only the older non-protected image.
• Leaves Docker’s default builder behavior intact; the larger margin absorbs BuildKit provenance/cache accounting.

Validation:

• toast lint
• toast validate_release