image: jangrewe/gitlab-ci-android
before_script:
  - apt-get --quiet update --yes
  - apt-get --quiet install --yes wget tar unzip lib32stdc++6 lib32z1 openjdk-8-jdk
  - echo y | ${ANDROID_HOME}/tools/android --silent update sdk --no-ui --all --filter android-24
  - echo y | ${ANDROID_HOME}/tools/android --silent update sdk --no-ui --all --filter platform-tools
  - echo y | ${ANDROID_HOME}/tools/android --silent update sdk --no-ui --all --filter build-tools-24.0.0
  - echo y | ${ANDROID_HOME}/tools/android --silent update sdk --no-ui --all --filter extra-android-m2repository
  - echo y | ${ANDROID_HOME}/tools/android --silent update sdk --no-ui --all --filter extra-google-google_play_services
  - echo y | ${ANDROID_HOME}/tools/android --silent update sdk --no-ui --all --filter extra-google-m2repository
  - chmod +x gradlew
build:
  script:
    - ./gradlew assembleDebug
  artifacts:
    paths:
    - app/build/outputs/apk/app-debug.apk

Change the build tools number to your compile target, and fork the docker image if you don't want to install everything every time.