* Update all submodules
* Ensure always using lastest docker image
* Allow passing in GEM_VERSION from env
This is useful to building test gems in a cache friendly way using:
`GEM_VERSION=$(git describe --tags 2>/dev/null | sed 's/-/./g' | sed
's/v//') bundle exec rake build_gem`
* Update submodules one last time
* Set version 6.0.0