Relies on legacy Docker links

[clue]

This image heavily relies on Docker's --link feature. This feature has been marked as "legacy" a while back (https://docs.docker.com/network/links/), but it's not quite "deprecated" yet (https://docs.docker.com/engine/deprecated/). This means that this feature is subject for removal in a future version of Docker at which point this image would be non-functional.

The same also affects the link setting in Docker Compose as per https://docs.docker.com/compose/compose-file/#links and https://docs.docker.com/compose/link-env-deprecated/. Only the legacy v1 format still exposes the required ENV variables. The current v3 docker-compose.yml does not expose the ENV variables anymore.