docker id는 대문자를 사용할 수 없는 것 같습니다.

Capital letter case (Error)

$ docker login

Username: YumaInaura
Password:

Error response from daemon: Get https://registry-1.docker.io/v2/: unauthorized: incorrect username or password

Lower letter case (Succeed)

$ docker login

Username: yumainaura
Password:

Login Succeeded

On Web Sign up

If you inputted capital case to Username,

This interface replace capital case with lower case silently.

Opps!

I think

Replace letter case automatically in docker services is better.
And I think "Username"expression is bad
Display "username"is better.