Dune pkg cacheable Docker build benchmark

This repo is a minimal OCaml project that demonstrates a cacheable Docker layer for Dune Package Management. It is intentionally tiny, but still builds the OCaml compiler and packages so you can see the Docker layer cache savings.

Blog post: https://makerprism.com/en/blog/a-practical-docker-cache-pattern-for-dune-package-management/

This is how I personally handle it. It is not a recommendation from the Dune team. I would love feedback or to hear how others handle this.

What this shows

• Dockerfile.unoptimized: copies the full repo first, so any source change invalidates the layer that builds the OCaml compiler and all dependencies.
• Dockerfile.optimized: copies only dune-project, runs dune pkg lock, and runs dune build @pkg-install, so the expensive compiler + dependency layer is cached. The real source code is copied afterward.

Run the benchmark

Cold builds:

time docker build -f Dockerfile.unoptimized .
time docker build -f Dockerfile.optimized .

Warm builds (after editing src/site.ml):

printf '%s\n' 'let () = print_endline "Hello again"' > src/site.ml
time docker build -f Dockerfile.unoptimized .
time docker build -f Dockerfile.optimized .

Benchmark results

Architecture:

• OS: Linux pop-os 6.17.9-76061709-generic
• CPU: AMD Ryzen 7 7840HS (16 threads)
• Arch: x86_64

Timings (local Docker, BuildKit enabled):

• Average warm rebuild (2 runs with actual source changes): unoptimized 109.7s, optimized 2.2s

Notes

• The cache boundary is the dune pkg lock + dune build @pkg-install layer.
• dune.lock is generated during the Docker build and is not checked in.
• This is Docker layer caching, not the Dune cache.