Update .gitlab-ci.yml

.gitlab-ci.yml

+image: i10git.cs.fau.de:5005/walberla/buildenvs/gcc:10
+
+stages:
+    - build
+    - test
+
+build:
+  stage: build
+  script:
+    - cd explicit-templates
+    - mkdir build
+    - cd build
+    - cmake ..
+    - make
+  artifacts:
+    paths:
+      - explicit-templates/build
+  rules:
+    - when: always
+
+# run tests using the binary built before
+test:
+  stage: test
+  script:
+    - cd explicit-templates/build
+  rules:
+    - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
+      when: manual