Generate Docker image

[rs028]

Use GitHub Actions to generate and store a Docker image whenever a new release is published.

[rs028]

@willdrysdale A new system to structure the model directory and use the build script (#416, #539) warrants a change of version number (v1.2 -> v1.3). So this seems like a good occasion to work on this, if you have the time. I can help with Actions, but I don't have experience with Docker,