For my case, the error was on “docker login” command.

The solution I found for my ubuntu: I downloaded the crt file via firefox (lock icon in the url adress bar) and save it :

~/mydomain:1234.crt

After that :

cp ~/mydomain:1234.crt /usr/local/share/ca-certificates/
update-ca-certificates
service docker restart

Source: https://stackoverflow.com/a/62404469/3550759