login: handle non-tty scenario consistently

[laurazard]

Running docker login in a non-interactive environment sometimes errors out if no username/password is provided.

Before:

--username	--password	Result
✅	✅	✅
❌	❌	Error: Cannot perform an interactive login from a non TTY device
✅	❌	Error: Cannot perform an interactive login from a non TTY device
❌	✅	hangs

After:

--username	--password	Result
✅	✅	✅
❌	❌	Error: Cannot perform an interactive login from a non TTY device
✅	❌	Error: Cannot perform an interactive login from a non TTY device
❌	✅	Error: Cannot perform an interactive login from a non TTY device

It's worth calling out a separate scenario – if there are previous, valid credentials, then running docker login with no username or password provided will use the previously stored credentials, and not error out.

cat ~/.docker/config.json
{
        "auths": {
                "https://index.docker.io/v1/": {
                        "auth": "xxxxxxxxxxx"
                }
        }
}
⭑ docker login 0>/dev/null
Authenticating with existing credentials...

Login Succeeded

- What I did

This PR makes this handling consistent, and also adds some tests.

- How I did it

Made the non-tty handling consistent by also erroring out when a user runs docker login --password bork (with no username) when stdin is not a tty.

Also applied the same non-interactive handling logic to the new web-based login flow, which means that now, if there are no prior credentials stored and a user runs docker login, instead of initiating the new web-based login flow, an error is returned.

- How to verify it

Run the added tests, or

• docker login 0>/dev/null
• docker login -u [username] -p [password] 0>/dev/null
• docker login -u [username] 0>/dev/null
• docker login -p [password] 0>/dev/null

- Description for the changelog

`docker login` now returns an error instead of hanging if called non-interactively with `--password` or `--password-stdin` but without `--user`.

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.73%. Comparing base (092b5f0) to head (bbb6e76).
Report is 548 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5401      +/-   ##
==========================================
- Coverage   60.94%   59.73%   -1.22%     
==========================================
  Files         304      345      +41     
  Lines       21346    23394    +2048     
==========================================
+ Hits        13010    13974     +964     
- Misses       7407     8450    +1043     
- Partials      929      970      +41     

[thaJeztah]

LGTM