docker5e.ffg.at has no CNAME records.
Question and response
Question
dig docker5e.ffg.at. NS
Answer
The response has no answer records.
Authority
ffg.at. 1788 SOA justin.ns.cloudflare.com. dns.cloudflare.com. 2397563220 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0