yarn with wercker - build step

yarn_with_wercker_build.yml

build:
  steps:
    - script:
      name: install yarn
      code: npm install -g yarn
    - script:
      name: report yarn version
      code: yarn --version
    - script:
      name: set yarn cache
      code: export YARN_CACHE=$WERCKER_CACHE_DIR/yarn
    - script:
      name: install dependencies
      code: HOME=$YARN_CACHE yarn
    - script:
      name: test
      code: yarn test:ci
    - script:
      name: coverage
      code: yarn coverage
    - script:
      name: lint
      code: yarn lint
    - script:
      name: build
      code: yarn build
    - script:
      name: copy build output
      code: cp -R build/ $WERCKER_OUTPUT_DIR
    - script:
      name: copy firebase config
      code: cp firebase.json $WERCKER_OUTPUT_DIR