Massively improve build performance

.gitlab-ci.yml

-image: openjdk:8-jdk
+image: k8r.eu/justjanne/android-sdk:27-27.0.3-26.1.1
 
 cache:
   paths:
     - .m2/
     - .gradle/
 
-variables:
-  ANDROID_COMPILE_SDK: "27"
-  ANDROID_BUILD_TOOLS: "27.0.3"
-  ANDROID_SDK_TOOLS: "26.1.1"
-
 before_script:
-  - apt-get --quiet update --yes
-  - apt-get --quiet install --yes wget tar unzip lib32stdc++6 lib32z1
-
-  - wget --quiet --output-document=android-sdk.zip https://dl.google.com/android/repository/sdk-tools-linux-3859397.zip
-  - unzip -q android-sdk.zip -d android-sdk-linux
-
-  - mkdir android-sdk-linux/licenses
-  - printf "8933bad161af4178b1185d1a37fbf41ea5269c55\nd56f5187479451eabf01fb78af6dfcb131a6481e" > android-sdk-linux/licenses/android-sdk-license
-  - printf "84831b9409646a918e30573bab4c9c91346d8abd" > android-sdk-linux/licenses/android-sdk-preview-license
-  - android-sdk-linux/tools/bin/sdkmanager --update > update.log
-  - android-sdk-linux/tools/bin/sdkmanager "platforms;android-${ANDROID_COMPILE_SDK}" "build-tools;${ANDROID_BUILD_TOOLS}" "extras;google;m2repository" "extras;android;m2repository" > installPlatform.log
-
-  - export ANDROID_HOME=$PWD/android-sdk-linux
-  - export PATH=$PATH:$PWD/android-sdk-linux/platform-tools/
+  - export ANDROID_HOME=/android-sdk-linux
+  - export PATH=$PATH:/android-sdk-linux/platform-tools/
   - chmod +x ./gradlew
 
   - echo $SIGNING_KEYSTORE | base64 -d > /root/signing.keystore
@@ -38,9 +21,10 @@ build:
   script:
     - echo $SIGNING_PROPERTIES > signing.properties
     - ./gradlew assembleRelease
+    - cp app/build/outputs/apk/release/*.apk .
   artifacts:
     paths:
-    - app/build/outputs/apk/release/
+    - *.apk
 
 unitTests:
   stage: test