What does it do?
This will provide composer. It is mainly used as a building block for other containers, but you can use this to install or run composer tasks from the container.
Commands you can run to use this.
Aliases you can use from the Docker DevTools aliases.
phpcs-drupal="docker_alias /app willhallonline/composer:alpine phpcs" phpcbf-drupal="docker_alias /app willhallonline/composer:alpine phpcbf"
docker pull willhallonline/composer:alpine
OR (if you want based on Debian rather than Alpine)
docker pull willhallonline/composer