OK - so I don’t know why, but I ran the manual upgrade process (git pull; ./launcher rebuild app) and it worked.
Lots of other activity like postgres upgrades etc came through as part of that manual upgrade.
My guess: something in that commit above, was not compatible with uglify-js or rails version or something, on the original container. It was basically an upgrade that required the ‘manual’ upgrade process.
Because the web admin interface let me upgrade docker_manager and then the discourse app itself, I assumed this would not be necessary (I know that sometimes it says an upgrade via the web admin cannot be done, and to do it manually… but that didn’t happen in this case, and probably should have…)
Thanks for your help anyway