Using PHP Code Sniffer

phpcs and phpcbf are readily available in the cli container.

Using as a custom fin command

It's recommended to see how to extend fin with custom commands first.

From your project's root folder (where the .docksal folder is) download the sample phpcs command

mkdir -p .docksal/commands
curl -fsSL https://raw.githubusercontent.com/docksal/docksal/master/examples/.docksal/commands/phpcs -o .docksal/commands/phpcs
chmod +x .docksal/commands/phpcs

Run fin phpcs docroot/sites/all/modules/custom or any path you want to run sniffer against.
See fin help phpcs for options.

Modify .docksal/commands/phpcs script as ou see fit.

A command for phpcbf can be created in a similar fashion.

Using directly

Instead of installing or creating custom commands you can use phpcs/phpcbf directly every time.
From your project's root folder run

fin run phpcs \
    --standard=Drupal -n \
    --extensions="php,module,inc,install,test,profile,theme" \
    --ignore="*.features.*,*.pages*.inc" \
    docroot/sites/all/modules/custom
fin run phpcbf \
    --standard=Drupal -n \
    --extensions="php,module,inc,install,test,profile,theme" \
    --ignore="*.features.*,*.pages*.inc" \
    docroot/sites/all/modules/custom