Track Docker Layers

[fiifikcbentil]

This PR introduces a new layers function val layers : t -> int that approximates the number of filesystem layers a given Dockerfile (Dockerfile.t) will produce.
The function inspects each instruction in the Dockerfile (Dockerfile.t) and counts only those that are guaranteed to create a new image layer in Docker, see: https://docs.docker.com/engine/storage/drivers/#images-and-layers. These are FROM, RUN, COPY, and ADD.
Being able to estimate the layer count from Dockerfile.t will help inform decisions regarding image size and build performance as well as help analyze the cost of adding new instructions

[MisterDA]

LGTM, thanks!

[MisterDA]

This could be improved for the case of multi-stage builds.

# let dockerfile =
    let open Dockerfile in
    (from ~alias:"build" "alpine:latest")
    @@ (run "echo hello > /world")
    @@ (from "alpine:latest")
    @@ (copy ~from:"build" ~link:true ~src:["/world"] ~dst:"/world") ()
  ;;
val dockerfile : Dockerfile.t = <abstr>
# Dockerfile.layers dockerfile;;
- : int = 4
# Format.printf "%a" Dockerfile.pp dockerfile;;
FROM alpine:latest AS build
RUN echo hello > /world

FROM alpine:latest
COPY --link --from=build [ "/world", "/world" ]
- : unit = ()

> docker build -t ocaml-dockerfile - <<EOF
FROM alpine:latest AS build
RUN echo hello > /world

FROM alpine:latest
COPY --link --from=build [ "/world", "/world" ]
EOF
> docker history ocaml-dockerfile
IMAGE          CREATED         CREATED BY                                      SIZE      COMMENT
ea6280f57fe7   7 minutes ago   COPY /world /world # buildkit                   6B        buildkit.dockerfile.v0
<missing>      2 months ago    CMD ["/bin/sh"]                                 0B        buildkit.dockerfile.v0
<missing>      2 months ago    ADD alpine-minirootfs-3.22.1-x86_64.tar.gz /…   8.31MB    buildkit.dockerfile.v0

That shows only two layers with a non-zero size. I think you should reset the count each time you encounter a `From.

[fiifikcbentil]

Thanks for catching that. You're right in the case of multi-stage builds, it makes sense to reset the counter whenever we hit a new FROM statement. I'll put in a follow-up change