RFE: Allow overriding the default image

[jmontleon]

Is your enhancement related to a problem? Please describe

When trying to work around #1074 I had to edit the extension code to use an alternate image. Initially I tried to just overwrite the run.sh with a slightly modified one, but since the image is pulled every time a new model service is started it was overwritten. This led to me pushing to a repo I have access to and editing the extension to use my image.

Describe the solution you'd like

I'd like there to be a way to point to a custom image for model services.

Describe alternatives you've considered

No response

Additional context

No response