[FR] Flutter cloud builder

## Detailed Feature Information

**Which builder is relevant to this request?**
None currently listed.

**Is your feature request related to a problem? Please describe.**
Not applicable.

**Describe the solution you'd like**
This request is for a cloud builder for flutter applications. The Dockerfile installs and configures the [flutter](https://docs.flutter.dev/reference/flutter-cli) executable as its [`CMD`](https://docs.docker.com/engine/reference/builder/#cmd).

**Describe alternatives you've considered**
Building and hosting privately on a Google Cloud project via Artifact Registry.

**Additional context**
I've already solved this and could provide it as a Pull Request to this repository.