fix: unify IsImageHerokuishBased with shell version

Really we should just spit this out into a bin and then have the shell version wrap the golang ones...

plugins/common/docker.go

@@ -274,7 +274,27 @@ func IsImageCnbBased(image string) bool {
 
 // IsImageHerokuishBased returns true if app image is based on herokuish
 func IsImageHerokuishBased(image string, appName string) bool {
-	output, err := DockerInspect(image, "{{range .Config.Env}}{{if eq . \"USER=herokuishuser\" }}{{println .}}{{end}}{{end}}")
+	if len(image) == 0 {
+		return false
+	}
+
+	if IsImageCnbBased(image) {
+		return true
+	}
+
+	dokkuAppUser := ""
+	if len(appName) != 0 {
+		b, err := PlugnTriggerOutput("config-get", []string{appName, "DOKKU_APP_USER"}...)
+		if err == nil {
+			dokkuAppUser = strings.TrimSpace(string(b))
+		}
+	}
+
+	if len(dokkuAppUser) == 0 {
+		dokkuAppUser = "herokuishuser"
+	}
+
+	output, err := DockerInspect(image, fmt.Sprintf("{{range .Config.Env}}{{if eq . \"USER=%s\" }}{{println .}}{{end}}{{end}}", dokkuAppUser))
 	if err != nil {
 		return false
 	}