pull version of Greenwood from package.json

Building on top off #1 (or maybe there is another way to do it) it would be great if the GREENWOOD_VERSION arg could be pull from the project's package.json at the time of building?

ARG GREENWOOD_VERSION=latest

Not sure if it's possible from just bash but would be handy for now to make sure the version of the @greenwood/cli is used in the container is the same as the one being used in the project's package.json.