First a short disclaimer: The Docker image sturai/ilias is not an official image from the ILIAS e.V., but a custom image provided by the company studer + raimann ag. The ILIAS community does not provide Docker images and there is no official support for it.
This being said, I will try to help you out if I can. If you are using Docker volumes, you should be able to just change the image from sturai/ilias:5.4-php7.3-apache to sturai/ilias:5.4-php7.2-apache and restart the container. The data in the Docker volumes will be preserved and mounted to the correct locations. There should be no need to copy data manually between containers.
If this does not help, try to have a look in the log. The image should print error messages to stdout (in line with Docker best practices).
This being said, I will try to help you out if I can. If you are using Docker volumes, you should be able to just change the image from sturai/ilias:5.4-php7.3-apache to sturai/ilias:5.4-php7.2-apache and restart the container. The data in the Docker volumes will be preserved and mounted to the correct locations. There should be no need to copy data manually between containers.
If this does not help, try to have a look in the log. The image should print error messages to stdout (in line with Docker best practices).