Build instructions using Docker

To get a binary built in your current workdir:

```console
$ docker run --rm -it -v "$(pwd):/workspace" rust:1.71.0-bookworm  # >= bookworm required to have gtk4 available in Ubuntu repos
$ git clone https://github.com/nik012003/ripdrag.git
$ cd ripdrag
$ apt update && apt install cargo libgtk-4-dev build-essential
$ cargo build
$ cp target/debug/ripdrag /workspace/
```