Configure Docker config.json for multiple users

[bertiewils]

These changes:

• Allow you to define a list of users who will have their config.json updated
• Set decent chown permissions on any created dirs/files
• Keep existing behaviour the same -ansible_env.USER is set in the defaults (which is the Ansible user), so user_data.home == ansible_user_dir

This is the most portable/readable way I could do it.

The alternative would be to have with_items: "{{ aws_ecr_cred_helper_users }}" on every task in tasks/configure-docker-json.yml.

Let me know your thoughts.

Tested on Ubuntu 22.04.