Missing icon cache is too aggressive

When a container is created and it doesn't have an icon, it will default to the question mark. However, if a valid image is defined later, even when the container is recreated and you hard refresh in browser, it's still the question mark.

If you look at DockerClient.php:

https://github.com/unraid/webgui/blob/8d40212ca286a39d6a1aea75768a76787afb46b2/emhttp/plugins/dynamix.docker.manager/include/DockerClient.php#L346

So then it gets persisted into state/plugins/dynamix.docker.manager/docker.json permanently unless manually changed. I think the code that exists for this case where it's missing the icon so it uses the fallback but then it gets a valid icon so then it changes to the correct icon is faulty;

https://github.com/unraid/webgui/blob/8d40212ca286a39d6a1aea75768a76787afb46b2/emhttp/plugins/dynamix.docker.manager/include/DockerClient.php#L340-L341

Because this will always evaluate as true since the placeholder image does exist, the check for a proper icon never runs again. The only solution is to manually edit the icon field in docker.json.

Might be related to #2691?

Start with emhttp/plugins/dynamix.docker.manager/include/DockerClient.php around lines 340-346 and inspect how the fallback icon is persisted in state/plugins/dynamix.docker.manager/docker.json. Reproduce the sequence with a container that first lacks an icon and later receives a valid image, then verify that recreation and a browser refresh show the valid icon without manually editing docker.json.