Docker able to mount filesystems.

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**Additional context**
It is currently not possible to mount filesystem inside of apprunner. Attempting to do so results in an error.

According to https://stackoverflow.com/a/42941866/1341731 docker has an option to give permission so that the container can mount filesystems using the docker wrapped mount command.

Please document how to accomplish this with app runner.

This is a show stopper for many projects.