add Dockerfile

Use latest slim python as base, create limited user, add code to wd,
install deps and show help by default but override the command at
runtime.

Build command:
```bash
docker build -t huami_token:`git describe` .
```

Run command:
```bash
docker run --rm -it huami_token:`git describe` -m xiaomi -b
```

Dockerfile

@@ -0,0 +1,8 @@
+FROM python:slim
+RUN useradd -U -m -d /app freedom
+USER freedom
+WORKDIR /app
+ADD --chown=freedom:freedom . .
+RUN pip install -r requirements.txt
+ENTRYPOINT ["python", "/app/huami_token.py"]
+CMD ["--help"]