WIP: docker-compose with override for mysql.

.docker/docker-compose.mysql.yml

+services:
+  sqldb:
+    image: mysql:8.0

.gitlab-ci.yml

@@ -185,8 +185,8 @@ test:
       - time docker load < /image-cache/mariadb.tar || true
       - docker pull $CI_REGISTRY_IMAGE
       - cd .docker
-        # here the server and the mysql backend docker are being started
-      - CAOSDB_TAG=$CAOSDB_TAG docker-compose  up -d
+      # here the server and the mysql backend docker are being started
+      - CAOSDB_TAG=$CAOSDB_TAG docker-compose -f docker-compose.yml -f docker-compose.mysql.yml  up -d
         # store versions of CaosDB parts
       - docker exec -u 0 -t docker_caosdb-server_1 cat /opt/caosdb/git/caosdb_pylib_commit > hash_pylib
       - docker exec -u 0 -t docker_caosdb-server_1 cat /opt/caosdb/git/caosdb_webui_commit > hash_webui