feat: add gitlab-ci

.gitlab-ci.yml

+stages:
+    - build
+
+variables:
+    MAVEN_OPTS: "-Dmaven.repo.local=$CI_PROJECT_DIR/.m2/repository"
+
+default:
+    cache:
+        key: $CI_COMMIT_REF_SLUG
+        paths:
+            - .m2/repository
+
+build-backend:
+    stage: build
+    image: maven:3.8.6-openjdk-18
+    before_script:
+        - mvn clean
+    script:
+        - mvn compile
+    artifacts:
+        paths:
+            - target/