cli/registry/login: Add the `--password-env` flag

Description

Add support for providing registry passwords via environment variables, which is particularly useful in CI pipelines. For example, GitLab stores registry passwords in the CI_REGISTRY_PASSWORD environment variable.

With the --password-env flag, authenticating to a registry is as simple as:

docker login --username "${CI_REGISTRY_USER}" --password-env "CI_REGISTRY_PASSWORD" "${CI_REGISTRY}"

Alternatives Considered

• Using docker login -p "${VAR}" with warning suppression

  • Shell history concerns don't apply with variable substitution
  • Tokens are often short-lived, reducing security concerns
  • Could introduce a way to suppress warnings via environment variables

• Passing passwords via STDIN (current recommended method):

  echo "${CI_REGISTRY_PASSWORD}" | docker login --username "${CI_REGISTRY_USER}" --password-stdin "${CI_REGISTRY}"
  

  Avoids warnings but adds complexity to command chains

Start by locating the Docker CLI's login command and its existing --password-stdin handling. Trace how command-line flags reach authentication, then verify that --password-env reads the named environment variable and supports the GitLab CI example without exposing the password.