Storing wallet.json inside a docker image is dangerous because by definition images are supposed to be disposable, user who did not save securely their keys outside of the image (wallet) will lose them forever.

If by image you mean container, you're correct. This is why I provided instructions for storing wallet.json files inside a named volume or volumes. Named volumes are not removed with the container, so unless the user deletes them manually, their keys are safe.

Although of course making additional backups is always a good idea. One can accomplish that by either finding where the volumes are stored on their machine and copying wallet.json file from there or using docker cp on a running wallet container.