Feature Request: Add compression level configuration option

Summary

Add a configuration option to control the gzip compression level for container image layers.

Motivation

Currently, ko uses gzip.BestSpeed (level 1) for layer compression via go-containerregistry. While this provides the fastest build times, it results in larger image sizes compared to higher compression levels.

In our production use case, we observed:

• Docker build (multi-stage): ~30MB
• ko build: ~33MB (~10% larger)

For users who prioritize image size over build speed (e.g., for bandwidth-constrained environments or storage costs), having the option to use higher compression levels would be valuable.

Proposed Solution

Add a compression or compressionLevel option to .ko.yaml:

# Option 1: Simple level (1-9)
compression:
  level: 9  # 1=BestSpeed, 9=BestCompression

# Option 2: Named presets
compression: best  # speed | default | best

And/or a CLI flag:

ko build --compression-level=9 ./cmd/app

Alternatives Considered

1. Use smaller base images: Already using distroless/chainguard static (~2-3MB)
2. Strip binaries: Already using -s -w ldflags
3. Upstream change in go-containerregistry: Would affect all users, not configurable

Additional Context

• This could be implemented by passing compression options to tarball.LayerFromOpener() or similar
• Default behavior should remain unchanged (BestSpeed) for backward compatibility
• Trade-off documentation would be helpful (build time vs image size)