Adding docker test to the README

... So that we can easily see when we break it

Author: Florent Clairambault

README.md

@@ -3,6 +3,7 @@
 ![Build](https://github.com/fclairamb/ftpserver/workflows/Build/badge.svg)
 ![Docker Image](https://github.com/fclairamb/ftpserver/workflows/Docker%20Image/badge.svg)
 ![Cross Build](https://github.com/fclairamb/ftpserver/workflows/Cross%20Build/badge.svg)
+![Docker test](https://github.com/fclairamb/ftpserver/workflows/Docker%20test/badge.svg)
 [![Go Report Card](https://goreportcard.com/badge/fclairamb/ftpserver)](https://goreportcard.com/report/fclairamb/ftpserver)
 [![GoDoc](https://godoc.org/github.com/fclairamb/ftpserver?status.svg)](https://godoc.org/github.com/fclairamb/ftpserver/server)