Have a look at the model configuration values for:
- container-image-metadata-url
- container-image-stream
There are some hoops to jump through here. To use your own custom images you need to serve a streams file that points to image blobs.
The default location, https://cloud-images.ubuntu.com/releases/streams/v1/index.json serves as an example.
The LP bug you linked only applies to the LXD provider, not to containers managed on Juju machines. In that case it is simple. Just add the image you want to use and add the appropriate alias.