www: Online tutorial doesn't load (not building)

[arlimus]

This occured for some builds on certain machines that don't have all the required modules installed (e.g. NodeJS).

Please make sure:

• Fail the build process if any shell command fails (e.g. npm install)
• Add sanity checks for build steps of: tutorial, docs, and www/middleman

[chris-rock]

Do we want to use a docker based build? eg. via https://hub.docker.com/_/node/

[arlimus]

@chris-rock That might be a great alternative. Instead of requiring node/npm we could just require docker, since it's a prerequisite for our tests anyway. I like the idea :)