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,
@@ -18,20 +17,15 @@
 #
 # 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-webui-testenv
-   # 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:latest
 
@@ -78,7 +72,7 @@ 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 test/docker