Fix instructions for building docker image

This fixes #84

Author: bitfehler

README.md

@@ -38,6 +38,8 @@ executable in `$GOPATH/bin`:
 
 You can build a Docker container with the included `docker` make target:
 
+    make promu
+    promu crossbuild -p linux/amd64 -p linux/arm64
     make docker
 
 This will not even require Go tooling on the host. See the included [docker