feat: add initial scheduled task implementation

This functionality enables users to manage a global crontab for the Dokku user that contains all scheduled tasks across all apps. Alternative schedulers can implement cron tasks as desired.

plugins/cron/src/commands/commands.go

@@ -0,0 +1,56 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+
+	"github.com/dokku/dokku/plugins/common"
+)
+
+const (
+	helpHeader = `Usage: dokku cron[:COMMAND]
+
+Manage scheduled tasks
+
+Additional commands:`
+
+	helpContent = `
+    cron:list <app>, List scheduled tasks for an app
+    cron:report [<app>] [<flag>], Display report about an app
+`
+)
+
+func main() {
+	flag.Usage = usage
+	flag.Parse()
+
+	cmd := flag.Arg(0)
+	switch cmd {
+	case "cron", "cron:help":
+		usage()
+	case "help":
+		command := common.NewShellCmd(fmt.Sprintf("ps -o command= %d", os.Getppid()))
+		command.ShowOutput = false
+		output, err := command.Output()
+
+		if err == nil && strings.Contains(string(output), "--all") {
+			fmt.Println(helpContent)
+		} else {
+			fmt.Print("\n    cron, Manage scheduled cron tasks\n")
+		}
+	default:
+		dokkuNotImplementExitCode, err := strconv.Atoi(os.Getenv("DOKKU_NOT_IMPLEMENTED_EXIT"))
+		if err != nil {
+			fmt.Println("failed to retrieve DOKKU_NOT_IMPLEMENTED_EXIT environment variable")
+			dokkuNotImplementExitCode = 10
+		}
+		os.Exit(dokkuNotImplementExitCode)
+	}
+}
+
+func usage() {
+	common.CommandUsage(helpHeader, helpContent)
+}