Fix actions badge in README.md (#163)

Author: majst01

README.md

@@ -1,6 +1,6 @@
 # go-ipam
 
-[![Actions](https://github.com/metal-stack/go-ipam/workflows/build/badge.svg)](https://github.com/metal-stack/go-ipam/actions)
+[![Actions](https://github.com/metal-stack/go-ipam/actions/workflows/docker.yml/badge.svg?branch=master)](https://github.com/metal-stack/go-ipam/actions)
 [![GoDoc](https://godoc.org/github.com/metal-stack/go-ipam?status.svg)](https://godoc.org/github.com/metal-stack/go-ipam)
 [![Go Report Card](https://goreportcard.com/badge/github.com/metal-stack/go-ipam)](https://goreportcard.com/report/github.com/metal-stack/go-ipam)
 [![codecov](https://codecov.io/gh/metal-stack/go-ipam/branch/master/graph/badge.svg)](https://codecov.io/gh/metal-stack/go-ipam)