build: ignore .devcontainer/ directory (angular#30417)

This makes it easier to experiment with VSCode's
[remote development using docker containers][1] feature.

In the future, we may check in the necessary files for users to use this
feature, but for now ignoring the directory makes it easier play around
and evaluate the feature.

[1]: https://code.visualstudio.com/docs/remote/containers

PR Close angular#30417

.gitignore

@@ -11,6 +11,7 @@ tools/gulp-tasks/cldr/cldr-data/
 pubspec.lock
 .c9
 .idea/
+.devcontainer
 .settings/
 .vscode/launch.json
 .vscode/settings.json