Speed up builds with Gradle caching

before_script:
  - export GRADLE_USER_HOME=cache/.gradle

...

cache:
  key: "$CI_COMMIT_REF_NAME" #optional: per branch caching, can be omitted
  paths:
    - $GRADLE_USER_HOME/caches/
    - $GRADLE_USER_HOME/wrapper/
    - $GRADLE_USER_HOME/build-cache/