Update Gradle.gitlab-ci.yml to use `gradle:alpine`

Merged Mark Pundsack requested to merge patch-2 into master

Closes https://gitlab.com/gitlab-org/gitlab-ci-yml/issues/24

This "works", but doesn't pass build artifacts from build to test, so is kinda dumb as it rebuilds again. It should either merge build and test into a single job, or store build artifacts.

Edited by Mark Pundsack