Remove overlaybd client path; warn on lazy_load_images

[jphenow]

Summary

The overlaybd/lazy-loading image feature is dead on the platform side:

• flyd no longer processes ImageRef.Layers — the overlaybd snapshotter was removed from hosts (nomad-firecracker#3341, merged 2025-05-22).
• rchab no longer serves the /flyio/v1/buildOverlaybdImage endpoint — the overlaybd build stages were removed (rchab#43).

Today, a customer with [experimental] lazy_load_images = true in fly.toml gets a silently broken deploy: flyctl converts the image, but the platform ignores the layers. After the rchab endpoint removal, the POST would 404 and fall through to a normal build via the existing terminal.Warnf — strictly better, but we shouldn't leave dead code and a misleading config flag around.

What changed

• Removed the buildOverlaybdImage function and the UseOverlaybd call path from dockerfile_builder.go
• Removed UseOverlaybd bool from the ImageOptions struct in resolver.go
• Removed the overlaybd-specific test case from daemon_url_test.go
• Removed the opts.UseOverlaybd plumbing from deploy_build.go
• Added a deprecation warning when [experimental] lazy_load_images = true is set in fly.toml
• Kept Experimental.LazyLoadImages in the config schema so existing fly.toml files still parse without error

Design decision: warn, don't error

Customers who have lazy_load_images = true in their fly.toml get a clear warning and a successful deploy — not a hard failure. This gives them a migration window to remove the stale flag without breaking their CI/CD pipelines.

Test plan

• go build ./... passes
• go vet ./internal/... passes
• go test ./internal/build/imgsrc/... passes
• Manual: set [experimental] lazy_load_images = true in a test app's fly.toml, run fly deploy --remote-only, verify:
  • No network request to /flyio/v1/buildOverlaybdImage
  • Warning about the flag is printed exactly once
  • Deploy succeeds as a normal image build
• Manual: remove the flag, verify no warning printed and deploy still works

[dangra]

@jphenow it needs to be go fmt. Other than that it looks good

[jphenow]

Bah! I setup prek but must’ve missed something. Will do