You have to set it via a environment variable:

UPLOAD_LIMIT (see https://hub.docker.com/_/phpmyadmin/)

To set max_upload_filesize in a phpMyAdmin Docker container, you need to adjust the PHP configuration used by the container.
Some phpMyAdmin Docker images support environment variables like PHP_UPLOAD_MAX_FILESIZE or PHP_POST_MAX_SIZE, but this is not always the case.

Check your image docs first, but most likely, you need to customize php.ini.