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/nullAuthenticating 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