Installs buildozer and dependencies, documents usage.
To build the image you first need to `cd` to where the `Dockerfile` is,
then run:
```sh
docker build --tag=buildozer .
```
Then you can use the container e.g. with:
```sh
docker run --volume "$(pwd)":/home/user/hostcwd buildozer --help
```
The above command mounts host current working directory in container
using `--volume` option.
Also added workaround for #625
