Read password from stdin for podman-hpc login

[EZoni]

Overview

Small change extracted from #432, which in turn was extracted from #428, now replaced by #439.

Avoid exposing REGISTRY_PASSWORD in the process list (ps output) by piping it via stdin instead of passing it as a --password argument.

Note that all documented examples at https://docs.podman.io/en/stable/markdown/podman-login.1.html (podman instead of podman-hpc) use --password-stdin.

Notes

The original suggestion in #432 and #428 had printf '%s\n' instead of echo to guard against passwords starting with -n, -e, or containing backslash sequences, but it should be robust enough to use echo in this case.