Remove legacy pack binary and heroku-20 pack selection logic

heroku-20 is EOL; the legacy pack v0.31.0 is no longer needed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: ipmb

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 
 * Builds using the `heroku-20` stack will now log a warning that the stack is end-of-life and should be upgraded to `heroku-24`
+* Removed support for `heroku-20` builds (previously required a legacy version of `pack`)
 
 ## [2.5.0] - 2025-10-21
 

Dockerfile

@@ -1,13 +1,9 @@
 FROM golang:1.25-alpine AS builder
 RUN apk add --no-cache curl
-# 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 && \
-    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 && \
+    mkdir -p /tmp/pack && \
+    cd /tmp/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"
 
@@ -16,7 +12,6 @@ COPY ./builder .
 RUN go build -o /go/bin/apppack-builder main.go
 
 FROM docker:27-dind
-COPY --from=builder /tmp/legacy-pack/pack /usr/local/bin/pack-legacy
-COPY --from=builder /tmp/current-pack/pack /usr/local/bin/pack
+COPY --from=builder /tmp/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

@@ -144,13 +144,7 @@ func (b *Build) buildWithDocker(config *containers.BuildConfig) error {
 func (b *Build) buildWithPack(config *containers.BuildConfig) error {
 	b.Log().Debug().Msg("pack config registry-mirrors")
 	builder := b.BuildpackBuilders()[0]
-	packBinary := "pack"
-	if builder == "heroku/buildpacks:20" {
-		// use legacy pack for heroku/buildpacks:20
-		packBinary = "pack-legacy"
-		b.Log().Debug().Msg(fmt.Sprintf("using legacy pack version for %s", builder))
-	}
-	cmd := exec.Command(packBinary, "config", "registry-mirrors", "add", "index.docker.io", "--mirror", DockerHubMirror)
+	cmd := exec.Command("pack", "config", "registry-mirrors", "add", "index.docker.io", "--mirror", DockerHubMirror)
 	if err := cmd.Run(); err != nil {
 		return err
 	}
@@ -170,7 +164,7 @@ func (b *Build) buildWithPack(config *containers.BuildConfig) error {
 	}
 	packArgs = append(packArgs, config.Image)
 	b.Log().Debug().Str("builder", builder).Str("buildpacks", buildpacks).Msg("building image")
-	cmd = exec.Command(packBinary, packArgs...)
+	cmd = exec.Command("pack", packArgs...)
 	out := io.MultiWriter(os.Stdout, config.LogFile)
 	cmd.Stdout = out
 	cmd.Stderr = out