List images within the Dockerfile via go programmatically using Moby Buildkit's official parser

main.go

package main

import (
	"fmt"
	"os"

	"github.com/google/go-containerregistry/pkg/authn"
	"github.com/google/go-containerregistry/pkg/name"
	"github.com/google/go-containerregistry/pkg/v1/remote"
	"github.com/moby/buildkit/frontend/dockerfile/parser"
)

func main() {
	f := os.Args[1]
	c, err := os.Open(f)
	if err != nil {
		panic(err)
	}

	defer c.Close()

	stages, err := parser.Parse(c)
	if err != nil {
		panic(err)
	}

	stageNames := map[string]struct{}{}
	for _, s := range stages.AST.Children {
		if s.Value == "from" {
			if _, ok := stageNames[s.Next.Value]; ok {
				continue
			}
			ref, err := name.ParseReference(s.Next.Value)
			if err != nil {
				panic(err)
			}

			img, err := remote.Image(ref, remote.WithAuthFromKeychain(authn.DefaultKeychain))
			if err != nil {
				panic(err)
			}
			dgst, err := img.Digest()
			if err != nil {
				panic(err)
			}
			fmt.Println(ref.Context().Digest(dgst.String()))
		}
	}
}