Pin docker version to 19.03.8

Our CI machine runs a Docker version which is too old otherwise. Hopefully to be
fixed in the near future

.gitlab-ci.yml

@@ -13,7 +13,7 @@ stages:
 build_docker_from_scratch:
   only:
     - schedules
-  image: docker:stable
+  image: docker:19.03.8
   stage: build_docker
   before_script:
     - ls
@@ -25,7 +25,7 @@ build_docker_from_scratch:
 build_docker_from_cache:
   except:
     - schedules
-  image: docker:stable
+  image: docker:19.03.8
   stage: build_docker
   before_script:
     - ls