Self-signed certificate with Kubernetes fix

In Go 1.15 there is common name deprecation for using Common name:
https://golang.google.cn/doc/go1.15#commonname

This causes the self-signed cert to be rejected by some Kubernetes platforms:
https://stackoverflow.com/questions/64814173/how-do-i-use-sans-with-openssl-instead-of-common-name

Fix is to add this field to the cert.

docs/insecure.md

@@ -68,6 +68,7 @@ This is more secure than the insecure registry solution.
 
     $ openssl req \
       -newkey rsa:4096 -nodes -sha256 -keyout certs/domain.key \
+      -addext "subjectAltName = DNS:myregistry.domain.com" \
       -x509 -days 365 -out certs/domain.crt
     ```