Add Actions Badge for Docker Image CI

Add
[![](https://github.com/tnir/mysqlclient/workflows/Docker%20Image%20CI/badge.svg?branch=master)](https://github.com/tnir/mysqlclient/actions)
with
```md
[![](https://github.com/tnir/mysqlclient/workflows/Docker%20Image%20CI/badge.svg?branch=master)](https://github.com/tnir/mysqlclient/actions)
```