feat: add ability to manage stacks on an app or global level

The previous lever for manipulating this was the DOKKU_IMAGE environment variable. While this is all well and good, it only works for CNB and is yet another `DOKKU_*` environment variable that need not exist.

While this does not add support for manipulating the CNB stack just yet, the groundwork is set for the future.

Closes #4306

plugins/buildpacks/src/commands/commands.go

@@ -24,6 +24,7 @@ Additional commands:`
     buildpacks:remove <app> <buildpack>, Remove a buildpack set on the app
     buildpacks:report [<app>] [<flag>], Displays a buildpack report for one or more apps
     buildpacks:set [--index 1] <app> <buildpack>, Set new app buildpack at a given position defaulting to the first buildpack if no index is specified
+    buildpacks:stacks-set <app> <stack>, Sets the stack of an app
 `
 )
 

plugins/buildpacks/src/subcommands/subcommands.go

@@ -57,6 +57,17 @@ func main() {
 		appName := args.Arg(0)
 		buildpack := args.Arg(1)
 		err = buildpacks.CommandSet(appName, buildpack, *index)
+	case "stacks-set":
+		args := flag.NewFlagSet("buildpacks:stacks-set", flag.ExitOnError)
+		global := args.Bool("global", false, "--global: set a global property")
+		args.Parse(os.Args[2:])
+		appName := args.Arg(0)
+		stack := args.Arg(1)
+		if *global {
+			appName = "--global"
+			stack = args.Arg(0)
+		}
+		err = buildpacks.CommandStacksSet(appName, stack)
 	default:
 		common.LogFail(fmt.Sprintf("Invalid plugin subcommand call: %s", subcommand))
 	}

plugins/buildpacks/src/triggers/triggers.go

@@ -18,6 +18,9 @@ func main() {
 
 	var err error
 	switch trigger {
+	case "buildpack-stack-name":
+		appName := flag.Arg(0)
+		err = buildpacks.TriggerBuildpackStackName(appName)
 	case "install":
 		err = buildpacks.TriggerInstall()
 	case "post-app-clone-setup":