WIP: setup with mysql 5.7

.docker/Dockerfile

-FROM debian:buster
+FROM mysql:8
 RUN apt-get update \
     && \
     apt-get install -y \

.gitlab-ci.yml

@@ -20,7 +20,7 @@
 #
 
 services:
-  - mysql:8
+  - mysql:5.7
 variables:
   DEPLOY_REF: dev
   CI_REGISTRY_IMAGE: $CI_REGISTRY/caosdb/src/caosdb-mysqlbackend/testenv:latest