Add command to docker build instructions
@@ -100,6 +100,7 @@ You can build a docker image locally with the following commands:
$ make promu
$ promu crossbuild -p linux/amd64
+ $ make npm_licenses
$ make common-docker-amd64
*NB* if you are on a Mac, you will need [gnu-tar](https://formulae.brew.sh/formula/gnu-tar).
