Use os.Hostname() instead of HOSTNAME env variable
Use os.Hostname() instead of HOSTNAME env variable

The assumption was made that the HOSTNAME environment variable
was available. While it is the case in Docker containers, it is
not in rkt containers. By using os.Hostname(), we have the
guarantee that we'll retrieve the hostname with both runtimes.

antoni/kubernetes-entrypoint,stackanetes/kubernetes-entrypoint