Apex domains cannot have CNAME records.
Question and response
Question
dig dobnet.cz. NS
Answer
dobnet.cz. 5 NS ns1.dobnet.cz.
dobnet.cz. 5 NS ns2.dobnet.cz.
Authority
The response has no authority records.
Additional
ns1.dobnet.cz. 5 A 139.28.116.3
ns2.dobnet.cz. 5 A 139.28.116.4
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0