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 you 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