add CI

.gitlab-ci.yml

+stages:
+   - test
+
+test:
+   stage: test
+   image: i10git.cs.fau.de:5005/walberla/buildenvs/gcc:11
+   script:
+      - git clone https://i10git.cs.fau.de/walberla/walberla.git
+      - mkdir build
+      - cd build
+      - cmake .. -DWALBERLA_DIR=../walberla
+      - cd apps/example_app/
+      - make -j 4
+      - ./ExampleApp
+   tags:
+      - docker