feat: implement getPhaseScript and scaffold out plugin triggers

plugins/app-json/appjson.go

@@ -0,0 +1,47 @@
+package appjson
+
+import (
+	"encoding/json"
+	"fmt"
+	"io/ioutil"
+	"os"
+
+	"github.com/dokku/dokku/plugins/common"
+)
+
+type AppJson struct {
+	Scripts struct {
+		Dokku string `json:"dokku"`
+	} `json:"scripts"`
+}
+
+// getPhaseScript extracts app.json from app image and returns the appropriate json key/value
+func getPhaseScript(appName string, image string, phase string) (string, error) {
+	appJsonFile, err := ioutil.TempFile(os.TempDir(), fmt.Sprintf("dokku-%s-%s", common.MustGetEnv("DOKKU_PID"), "getPhaseScript"))
+	if err != nil {
+		return "", fmt.Errorf("Cannot create temporary file: %v", err)
+	}
+
+	defer os.Remove(appJsonFile.Name())
+
+	common.CopyFromImage(appName, image, "app.json", appJsonFile.Name())
+	if !common.FileExists(appJsonFile.Name()) {
+		return "", nil
+	}
+
+	b, err := ioutil.ReadAll(appJsonFile)
+	if err != nil {
+		return "", fmt.Errorf("Cannot read app.json file: %v", err)
+	}
+
+	var appJson AppJson
+	if err = json.Unmarshal(b, &appJson); err != nil {
+		return "", fmt.Errorf("Cannot parse app.json: %v", err)
+	}
+
+	return appJson.Scripts.Dokku, nil
+}
+
+func executeScript(appName string, imageTag string, phase string) error {
+	return nil
+}