Before this change, any attempt to run the docker image lead to:

```
/docker-entrypoint.sh: line 59: /usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java: No such file or directory
```

For unknown reasons, the jre installation prefix of the
`openjdk:8-jre-slim` image has changed. This PR is updating it, making
the image functional again.