How to use command stdin in Ansible?

If you want to use the stdin argument to the command module, take a look at the docs, which show examples using other options such as creates, which looks like this:

# You can also use the 'args' form to provide the options.
- name: This command will change the working directory to somedir/ and will only run when /path/to/database doesn't exist.
  command: /usr/bin/make_database.sh arg1 arg2
  args:
    chdir: somedir/
    creates: /path/to/database

For your use case, you would want:

- name: Log into Docker registry
  command: docker login --username "{{ docker_registry_username }}" --password-stdin
  args:
    stdin: "{{ docker_registry_password }}"

Your first attempt failed because you were setting stdin as a key at the task level (like when or ignore_errors, etc), when you actually want it to be an argument to the command module.

Your second attempt failed because it wasn't valid YAML.

Why do you use command/shell to log into Docker registry? Ansible has docker_login module for that: https://docs.ansible.com/ansible/latest/modules/docker_login_module.html it's in preview, from community, but it works.

You can create vault file with sensitive data.

Sounds like you want to use the Ansible expect module. See https://docs.ansible.com/ansible/latest/modules/expect_module.html.

Hope this helps.