: add .gitlab-ci.yaml

.gitlab-ci.yml

+image: gradle:alpine
+
+before_script:
+  - GRADLE_USER_HOME="$(pwd)/.gradle"
+  - export GRADLE_USER_HOME
+
+build:
+  stage: build
+  script: gradle assemble
+  artifacts:
+    untracked: false
+    when: on_success
+    expire_in: 30 days
+    paths:
+      - build/libs/*.jar
\ No newline at end of file