Enable build chaching for s2i local build

Signed-off-by: Matej Vašek <[email protected]>

pkg/builders/s2i/builder.go

@@ -3,6 +3,8 @@ package s2i
 import (
 	"archive/tar"
 	"context"
+	"crypto/sha1"
+	"encoding/hex"
 	"errors"
 	"fmt"
 	"io"
@@ -256,6 +258,12 @@ func (b *Builder) Build(ctx context.Context, f fn.Function, platforms []fn.Platf
 	// s2i apparently is not excluding the files in --as-dockerfile mode
 	exclude := regexp.MustCompile(cfg.ExcludeRegExp)
 
+	// patch dockerfile to using cache mount
+	err = patchDockerfile(cfg.AsDockerfile, f)
+	if err != nil {
+		return err
+	}
+
 	const up = ".." + string(os.PathSeparator)
 	go func() {
 		tw := tar.NewWriter(pw)
@@ -356,6 +364,20 @@ func (b *Builder) Build(ctx context.Context, f fn.Function, platforms []fn.Platf
 	return jsonmessage.DisplayJSONMessagesStream(resp.Body, out, fd, isTerminal, nil)
 }
 
+func patchDockerfile(path string, f fn.Function) error {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return err
+	}
+	re := regexp.MustCompile(`RUN (.*assemble)`)
+	s := sha1.Sum([]byte(f.Root))
+	mountCmd := "--mount=type=cache,target=/tmp/artifacts/,uid=1001,id=" + hex.EncodeToString(s[:8])
+	replacement := fmt.Sprintf("RUN %s \\\n    $1", mountCmd)
+	newDockerFileStr := re.ReplaceAllString(string(data), replacement)
+
+	return os.WriteFile(path, []byte(newDockerFileStr), 0644)
+}
+
 func s2iScriptURL(ctx context.Context, cli DockerClient, image string) (string, error) {
 	img, _, err := cli.ImageInspectWithRaw(ctx, image)
 	if err != nil {