fix gitlab ci by setting docker versions

.gitlab-ci.yml

 #
-# ** header v3.0
 # This file is a part of the CaosDB Project.
 #
 # Copyright (C) 2018 Research Group Biomedical Physics,
@@ -19,26 +18,22 @@
 # You should have received a copy of the GNU Affero General Public License
 # along with this program. If not, see <https://www.gnu.org/licenses/>.
 #
-# ** end header
-#
 
 variables:
   CI_REGISTRY_IMAGE: $CI_REGISTRY/caosdb-mysqlbackend-testenv:latest
-  # With dind the docker deamon is available on the network 
-  DOCKER_HOST: tcp://docker:2375/
   # When using dind, it's wise to use the overlayfs driver for
   # improved performance.
   DOCKER_DRIVER: overlay2
 
 services:
-  - docker:dind
+  - docker:19.03-dind
 
 image: $CI_REGISTRY_IMAGE
 stages:
   - setup
   - deploy
 
-# Trigger integration tests
+# Trigger building of server image and integration tests
 trigger_build:
   tags: [ docker ]
   stage: deploy
@@ -53,7 +48,7 @@ trigger_build:
 # Build a docker image in which tests for this repository can run
 build-testenv:
   tags: [ docker ]
-  image: docker:latest
+  image: docker:19.03
   stage: setup
   script: 
     - cd .docker