add Yarn to build-image

[fool]

https://github.com/yarnpkg/yarn

[fool]

while you're in there, it would be awesome to get #24 and #22 handled as well @rybit :)

[t-richards]

Arguably, the most important feature of yarn is that it installs dependencies over 9000% faster than npm. Unfortunately, due to the fact that npm install is run automatically by run-build.sh, it kind of defeats the purpose of having yarn available in the first place, IMO.

It would be nice to have some way to opt-out of this default behavior. Maybe an environment variable SKIP_NPM=true that would tell run-build.sh to skip the npm install portion of the script?

I'd be happy to write up a PR if you're willing to entertain this as a possibility.

[biilmann]

Our plan with YARN is to use it instead of npm install whenever we detect a yarn.lock file

[hhsnopek]

Ayye! This added a couple days ago in 449895f 😄