README: Add Docker Hub pull count badge

```
[![Docker Pulls](https://badgen.net/docker/pulls/absaoss/spline-rest-server?icon=docker&label=pulls)](https://hub.docker.com/r/absaoss/spline-rest-server/)
```

[![Docker Pulls](https://badgen.net/docker/pulls/absaoss/spline-rest-server?icon=docker&label=pulls)](https://hub.docker.com/r/absaoss/spline-rest-server/)