Fix docker push: Add login with GitLab-provided credentials

.gitlab-ci.yml

@@ -4,4 +4,6 @@ build:
   stage: build
   script:
     - docker image build -t registry.git.rwth-aachen.de/acplt/rte-tools/rte-build-docker-image/image .
+    - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
     - docker push registry.git.rwth-aachen.de/acplt/rte-tools/rte-build-docker-image/image
+    - docker logout $CI_REGISTRY