fix: remove buildpacks from dockerfile test app and add tests for app.json buildpacks

The buildpacks entry in tests/apps/dockerfile/app.json caused
the post-extract trigger to write a .buildpacks file during
Dockerfile-based deploys, breaking builder detection. Remove
the entry and add Go unit tests for getBuildpacks/validBuildpackURL,
a bats integration test for deploying with app.json buildpacks,
and documentation for the new feature.

plugins/buildpacks/buildpacks_test.go

@@ -0,0 +1,141 @@
+package buildpacks
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	. "github.com/onsi/gomega"
+)
+
+func setupTestEnvironment(t *testing.T) string {
+	t.Helper()
+	tmpDir, err := os.MkdirTemp("", "buildpacks-test")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	os.Setenv("DOKKU_LIB_ROOT", tmpDir)
+	os.Setenv("PLUGIN_PATH", "/var/lib/dokku/plugins")
+	os.Setenv("PLUGIN_ENABLED_PATH", "/var/lib/dokku/plugins/enabled")
+
+	return tmpDir
+}
+
+func teardownTestEnvironment(tmpDir string) {
+	os.RemoveAll(tmpDir)
+}
+
+func TestValidBuildpackURL(t *testing.T) {
+	RegisterTestingT(t)
+
+	url, err := validBuildpackURL("")
+	Expect(err).To(HaveOccurred())
+	Expect(err.Error()).To(ContainSubstring("Must specify a buildpack"))
+
+	url, err = validBuildpackURL("heroku/nodejs")
+	Expect(err).NotTo(HaveOccurred())
+	Expect(url).To(Equal("https://github.com/heroku/heroku-buildpack-nodejs.git"))
+
+	url, err = validBuildpackURL("heroku/python")
+	Expect(err).NotTo(HaveOccurred())
+	Expect(url).To(Equal("https://github.com/heroku/heroku-buildpack-python.git"))
+
+	url, err = validBuildpackURL("heroku-community/apt")
+	Expect(err).NotTo(HaveOccurred())
+	Expect(url).To(Equal("https://github.com/heroku/heroku-buildpack-apt.git"))
+
+	url, err = validBuildpackURL("https://github.com/heroku/heroku-buildpack-nodejs.git")
+	Expect(err).NotTo(HaveOccurred())
+	Expect(url).To(Equal("https://github.com/heroku/heroku-buildpack-nodejs.git"))
+
+	url, err = validBuildpackURL("https://github.com/heroku/heroku-buildpack-nodejs")
+	Expect(err).NotTo(HaveOccurred())
+	Expect(url).To(Equal("https://github.com/heroku/heroku-buildpack-nodejs"))
+
+	url, err = validBuildpackURL("git://github.com/heroku/heroku-buildpack-nodejs.git")
+	Expect(err).NotTo(HaveOccurred())
+	Expect(url).To(Equal("git://github.com/heroku/heroku-buildpack-nodejs.git"))
+
+	_, err = validBuildpackURL("nodejs")
+	Expect(err).To(HaveOccurred())
+	Expect(err.Error()).To(ContainSubstring("Invalid buildpack specified"))
+
+	_, err = validBuildpackURL("/nodejs")
+	Expect(err).To(HaveOccurred())
+	Expect(err.Error()).To(ContainSubstring("Invalid buildpack specified"))
+}
+
+func TestGetBuildpacksEmpty(t *testing.T) {
+	RegisterTestingT(t)
+	tmpDir := setupTestEnvironment(t)
+	defer teardownTestEnvironment(tmpDir)
+
+	buildpacks, err := getBuildpacks("test-app")
+	Expect(err).NotTo(HaveOccurred())
+	Expect(buildpacks).To(BeEmpty())
+}
+
+func TestGetBuildpacksFromProperties(t *testing.T) {
+	RegisterTestingT(t)
+	tmpDir := setupTestEnvironment(t)
+	defer teardownTestEnvironment(tmpDir)
+
+	propDir := filepath.Join(tmpDir, "config", "buildpacks", "test-app")
+	Expect(os.MkdirAll(propDir, 0755)).To(Succeed())
+	Expect(os.WriteFile(filepath.Join(propDir, "buildpacks"), []byte("https://github.com/heroku/heroku-buildpack-nodejs.git\nhttps://github.com/heroku/heroku-buildpack-python.git\n"), 0644)).To(Succeed())
+
+	buildpacks, err := getBuildpacks("test-app")
+	Expect(err).NotTo(HaveOccurred())
+	Expect(buildpacks).To(HaveLen(2))
+	Expect(buildpacks[0]).To(Equal("https://github.com/heroku/heroku-buildpack-nodejs.git"))
+	Expect(buildpacks[1]).To(Equal("https://github.com/heroku/heroku-buildpack-python.git"))
+}
+
+func TestGetBuildpacksFromAppJSON(t *testing.T) {
+	RegisterTestingT(t)
+	tmpDir := setupTestEnvironment(t)
+	defer teardownTestEnvironment(tmpDir)
+
+	appJSONDir := filepath.Join(tmpDir, "data", "app-json", "test-app")
+	Expect(os.MkdirAll(appJSONDir, 0755)).To(Succeed())
+	Expect(os.WriteFile(filepath.Join(appJSONDir, "app.json"), []byte(`{"buildpacks": [{"url": "https://github.com/heroku/heroku-buildpack-nodejs.git"}]}`), 0644)).To(Succeed())
+
+	buildpacks, err := getBuildpacks("test-app")
+	Expect(err).NotTo(HaveOccurred())
+	Expect(buildpacks).To(HaveLen(1))
+	Expect(buildpacks[0]).To(Equal("https://github.com/heroku/heroku-buildpack-nodejs.git"))
+}
+
+func TestGetBuildpacksPropertiesOverrideAppJSON(t *testing.T) {
+	RegisterTestingT(t)
+	tmpDir := setupTestEnvironment(t)
+	defer teardownTestEnvironment(tmpDir)
+
+	propDir := filepath.Join(tmpDir, "config", "buildpacks", "test-app")
+	Expect(os.MkdirAll(propDir, 0755)).To(Succeed())
+	Expect(os.WriteFile(filepath.Join(propDir, "buildpacks"), []byte("https://github.com/heroku/heroku-buildpack-ruby.git\n"), 0644)).To(Succeed())
+
+	appJSONDir := filepath.Join(tmpDir, "data", "app-json", "test-app")
+	Expect(os.MkdirAll(appJSONDir, 0755)).To(Succeed())
+	Expect(os.WriteFile(filepath.Join(appJSONDir, "app.json"), []byte(`{"buildpacks": [{"url": "https://github.com/heroku/heroku-buildpack-nodejs.git"}]}`), 0644)).To(Succeed())
+
+	buildpacks, err := getBuildpacks("test-app")
+	Expect(err).NotTo(HaveOccurred())
+	Expect(buildpacks).To(HaveLen(1))
+	Expect(buildpacks[0]).To(Equal("https://github.com/heroku/heroku-buildpack-ruby.git"))
+}
+
+func TestGetBuildpacksAppJSONEmptyArray(t *testing.T) {
+	RegisterTestingT(t)
+	tmpDir := setupTestEnvironment(t)
+	defer teardownTestEnvironment(tmpDir)
+
+	appJSONDir := filepath.Join(tmpDir, "data", "app-json", "test-app")
+	Expect(os.MkdirAll(appJSONDir, 0755)).To(Succeed())
+	Expect(os.WriteFile(filepath.Join(appJSONDir, "app.json"), []byte(`{"buildpacks": []}`), 0644)).To(Succeed())
+
+	buildpacks, err := getBuildpacks("test-app")
+	Expect(err).NotTo(HaveOccurred())
+	Expect(buildpacks).To(BeEmpty())
+}

plugins/buildpacks/go.mod

@@ -11,6 +11,7 @@ require (
 require (
 	github.com/alexellis/go-execute/v2 v2.2.1 // indirect
 	github.com/fatih/color v1.19.0 // indirect
+	github.com/google/go-cmp v0.7.0 // indirect
 	github.com/hashicorp/errwrap v1.0.0 // indirect
 	github.com/hashicorp/go-multierror v1.1.1 // indirect
 	github.com/kballard/go-shellquote v0.0.0-20180428030007-95032a82bc51 // indirect
@@ -18,15 +19,19 @@ require (
 	github.com/mattn/go-colorable v0.1.14 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
 	github.com/melbahja/goph v1.5.0 // indirect
+	github.com/onsi/gomega v1.39.1 // indirect
 	github.com/otiai10/copy v1.14.1 // indirect
 	github.com/otiai10/mint v1.6.3 // indirect
 	github.com/pkg/errors v0.9.1 // indirect
 	github.com/pkg/sftp v1.13.10 // indirect
 	github.com/ryanuber/columnize v2.1.2+incompatible // indirect
 	github.com/tailscale/hujson v0.0.0-20250605163823-992244df8c5a // indirect
+	go.yaml.in/yaml/v3 v3.0.4 // indirect
 	golang.org/x/crypto v0.50.0 // indirect
+	golang.org/x/net v0.52.0 // indirect
 	golang.org/x/sync v0.20.0 // indirect
 	golang.org/x/sys v0.43.0 // indirect
+	golang.org/x/text v0.36.0 // indirect
 	k8s.io/utils v0.0.0-20240102154912-e7106e64919e // indirect
 )
 

plugins/buildpacks/go.sum

@@ -55,6 +55,7 @@ golang.org/x/term v0.42.0 h1:UiKe+zDFmJobeJ5ggPwOshJIVt6/Ft0rcfrXZDLWAWY=
 golang.org/x/term v0.42.0/go.mod h1:Dq/D+snpsbazcBG5+F9Q1n2rXV8Ma+71xEjTRufARgY=
 golang.org/x/text v0.36.0 h1:JfKh3XmcRPqZPKevfXVpI1wXPTqbkE5f7JA92a55Yxg=
 golang.org/x/text v0.36.0/go.mod h1:NIdBknypM8iqVmPiuco0Dh6P5Jcdk8lJL0CUebqK164=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
 gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
 k8s.io/utils v0.0.0-20240102154912-e7106e64919e h1:eQ/4ljkx21sObifjzXwlPKpdGLrCfRziVtos3ofG/sQ=