cli/registry: support password dash stdin

[agirault]

What

Support for docker login --password - and docker login -p - to read the password or PAT from stdin, matching the existing --password-stdin behavior, and following the common CLI convention that - means stdin.

The tradeoff is that - can no longer be used as a literal password value through --password.

Validation

make -f docker.Makefile test-unit

&

docker run --rm -v "$PWD":/src -w /src golang:1.25 ./scripts/with-go-mod.sh go test -mod=mod ./cli/command/registry -run 'TestRunLogin/password_dash|TestLoginHelpDocumentsPasswordDash|TestLoginValidateFlags/(password_dash|short_password_dash|password_stdin_and_password_dash|conflicting_options)'

Note

• I searched for prior Docker CLI issues and PRs rejecting this behavior and did not find one.
• Figured an issue was not needed for this and that it could be discussed in an MR instead given the low cost to just write it.
• No strong feelings if rejected.