Pin neo4j docker image versions in provided Dockerfiles

[jeremypuchta]

The jQA dashboard does not work with the latest version of Neo4j. The latest version of Neo4j, currently version 4.0.5, is used as the base image in the Dockerfiles provided. Therefore, it's necessary to pin the used Neo4j docker images in the provided Dockerfiles neo4j-junit and neo4j-petclinic as it's best practice (https://nickjanetakis.com/blog/docker-tip-18-please-pin-your-docker-image-versions).

It still has to be investigated, which is the last still working version but probably the latest version of Neo4j 3.5 should be fine. On my computer, version 3.5.3 is working well.