Docker image can fail to build for Server < 7.0.0

Linux installers for older versions of Server used a method to detect whether they were running in a container. This was used to decide whether to delegate to systemd to start/stop Server or whether to invoke those commands directly (since systemd isn't available in containers). Unfortunately this heuristic didn't work in some environments, most notably in the automated testing of Dockerfiles that we proposed for our Official Docker Image.

I fixed this in 7.0.0 (CBD-3908) and the Docker team accepted our earlier images with some manual testing. However, just recently even their manual testing started failing, presumably due to some updates on their end. It appears that Docker Hub likely will not be able to build any < 7.0.0 Server Dockerfiles at all - this would include automated security updates done by Docker Hub.