Thanks for the help!
I get it now. I was confused at first thinking that discourse and docker_manager were two alternative ways to update Discourse, and didn’t realize they were completely separate things and that updating docker_manager was a prerequisite to updating discourse 
Here’s what I’ve done:
- I pressed Update on
docker_manager, bringing it up to date as shown here:
- Since the Update button for updating Discourse was then enabled, I ran that.
And now my forum is up to date 
For now I’ve decided to stay with the default install but may look into switching to stable in future.