feat: image-optimize

[Harshal96]

Summary

Adds an explicit image_optimize rule for image-wide layer optimization.

This lets users rewrite every available layer in an image_manifest or image_index, including eagerly materialized base-image layers, and enables whole-image eStargz output without changing the default shallow-pull behavior.

Why

Issue #421 asks about applying eStargz at the image/base-image level. Instead of making image_manifest or image_index implicitly download and rewrite base layers, this adds a separate opt-in rule. That keeps shallow pulls cheap by default while giving users a clear path when they intentionally want all layers optimized.

Changes

• Adds image_optimize to @rules_img//img:image.bzl
• Supports both single-platform manifests and multi-platform indexes
• Recompresses every available layer using selected compression settings
• Supports estargz = "enabled" for whole-image eStargz conversion
• Fails clearly if any input layer blob is unavailable or shallow
• Preserves source image manifest/config/index metadata while replacing layer descriptors and config rootfs.diff_ids
• Preserves existing layer annotations during recompression, while replacing stale eStargz-generated annotations
• Regenerates docs/image.md
• Adds toolchain and e2e coverage

Test Plan

• bazel test @rules_img_tool//cmd/img:test
• bazel test //tests/img_toolchain:img_toolchain_test
• bazel test //util:buildifier.check
• cd e2e/generic && bazel test //:optimize_tests

[malt3]

This looks very promising at first glance. I'll need some more time for an in-depth review. Thank you for your work!