Use newer pack for all but heroku/buildpacks:20

Author: ipmb

Dockerfile

@@ -1,9 +1,13 @@
-FROM golang:1.22-alpine as builder
+FROM golang:1.22-alpine AS builder
 RUN apk add --no-cache curl
-# last pack version that supports heroku/buildpacks:20 builder
-ENV PACK_VER=0.31.0
+# Install current `pack` and v0.31.0, the last pack version that supports heroku/buildpacks:20 builder
+ENV PACK_VER=0.38.0
 RUN set -ex && \
-    cd /tmp && \
+    mkdir -p /tmp/legacy-pack /tmp/current-pack && \
+    cd /tmp/legacy-pack && \
+    curl -sLO "https://github.com/buildpacks/pack/releases/download/v0.31.0/pack-v0.31.0-linux.tgz" && \
+    tar xvzf "pack-v0.31.0-linux.tgz" && \
+    cd /tmp/current-pack && \
     curl -sLO "https://github.com/buildpacks/pack/releases/download/v$PACK_VER/pack-v$PACK_VER-linux.tgz" && \
     tar xvzf "pack-v$PACK_VER-linux.tgz"
 
@@ -12,6 +16,7 @@ COPY ./builder .
 RUN go build -o /go/bin/apppack-builder main.go
 
 FROM docker:26-dind
-COPY --from=builder /tmp/pack /usr/local/bin/pack
+COPY --from=builder /tmp/legacy-pack/pack /usr/local/bin/pack-legacy
+COPY --from=builder /tmp/current-pack/pack /usr/local/bin/pack
 RUN apk add --no-cache git
 COPY --from=builder /go/bin/apppack-builder /usr/local/bin/apppack-builder

builder/build/build.go

@@ -137,11 +137,17 @@ func (b *Build) buildWithDocker(config *containers.BuildConfig) error {
 
 func (b *Build) buildWithPack(config *containers.BuildConfig) error {
 	b.Log().Debug().Msg("pack config registry-mirrors")
-	cmd := exec.Command("pack", "config", "registry-mirrors", "add", "index.docker.io", "--mirror", DockerHubMirror)
+	builder := b.BuildpackBuilders()[0]
+	cmdName := "pack"
+	if builder == "heroku/buildpacks:20" {
+		// use legacy pack for heroku/builder:20
+		cmdName = "pack-legacy"
+		b.Log().Debug().Msg("using legacy pack version for heroku/buildpacks:20")
+	}
+	cmd := exec.Command(cmdName, "config", "registry-mirrors", "add", "index.docker.io", "--mirror", DockerHubMirror)
 	if err := cmd.Run(); err != nil {
 		return err
 	}
-	builder := b.BuildpackBuilders()[0]
 	buildpacks := strings.Join(b.AppJSON.GetBuildpacks(), ",")
 	packArgs := []string{
 		"build",