How docker sets the default source

in previous versions:
docker Version: 1.12.6
can replace the default source of docker with your own private source.

on the new version:
docker version: 18.03.0-ce
seems unable to set its own private source as the default, and always uses docker.io

.

there are too many configurations on the Internet, which one is right?

Mar.19,2021
MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1b3e533-2b94f.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b3e533-2b94f.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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