add Dockerfile from https://yunohost.org/#/docker

Loving your project! Might it be an idea to ship the Dockerfile inside the repo? Then you can version it along, whenever a change in install_script causes Dockerfile to change, you can do that in the same update, and make sure the docker builds always works with the right version of install_script.

This would change [the "Build the YunoHost container" instructions](https://yunohost.org/#/docker) from:

```bash
docker build -t yunohost:init https://raw.githubusercontent.com/YunoHost/Kremlin/master/docker/Dockerfile
```

to:

```bash
git clone https://github.com/YunoHost/install_script
docker build -t yunohost:init install_script
```

Downside is you add a requirement for git on the host system, but I think it's worth it.