The latter is more portable, while the former only works on systems where /bin/bash exists (or is symlinked appropriately).
Currently if container under test has multiple IP addresses, the `docker_ip` function from `docker.sh` will return a gibberish. This patch makes it return the first address found. Additionally, I apply shellcheck on `docker.sh`.
Test servers are implemented by docker containers and run real servers for rclone to test against.