Fix the sandbox check to also work for docker/podman containers (#78)

* fix the sandbox check to also work for docker/podman containers

* check for non-empty value of "container" env var

Author: sumwale

packages/tray_manager/lib/src/helpers/sandbox.dart

@@ -1,7 +1,9 @@
 import 'dart:io';
 
-/// Returns `true` if the app is running in a sandbox, eg. Flatpak or Snap.
+/// Returns `true` if the app is running in a sandbox, eg. Flatpak, Snap, Docker, Podman.
 bool runningInSandbox() {
   return Platform.environment.containsKey('FLATPAK_ID') ||
-      Platform.environment.containsKey('SNAP');
+      Platform.environment.containsKey('SNAP') ||
+      (Platform.environment['container']?.isNotEmpty == true) ||
+      FileSystemEntity.isFileSync('/.dockerenv');
 }