fix: improved offline support

Closes coop-cloud/organising#471.

cli/app/backup.go

@@ -41,6 +41,7 @@ var appBackupCommand = cli.Command{
 	ArgsUsage: "<domain> [<service>]",
 	Flags: []cli.Flag{
 		internal.DebugFlag,
+		internal.OfflineFlag,
 	},
 	Before:       internal.SubCommandBefore,
 	BashComplete: autocomplete.AppNameComplete,
@@ -71,8 +72,8 @@ This file is a compressed archive which contains all backup paths. To see paths,
 This single file can be used to restore your app. See "abra app restore" for more.
 `,
 	Action: func(c *cli.Context) error {
-		app := internal.ValidateApp(c)
-		conf := runtime.New()
+		conf := runtime.New(runtime.WithOffline(internal.Offline))
+		app := internal.ValidateApp(c, conf)
 
 		cl, err := client.New(app.Server)
 		if err != nil {