README.md doesn't mention that Docker is required for build

README.md on github says:

If you'd like to build and install the whole project from the source distribution, you may need some additional tools installed in your system. In a Debian-based distribution:

sudo apt-get install \
openjdk-8-jdk \
python-setuptools \
python-pip \
virtualenv

That's correct as far as it goes, but it's incomplete. .gradlew will fail in task ':website:buildDockerImage' unless docker is installed and running.