Thanks @sam. I’ve done that and I am able to build the new version now. Fortunately, this was all on beta so no harm done 
I’m not sure it is “hackery” to want a repeatable build though 
The thing I am trying to get here is the ability to roll back to a known good version. Suppose that I run ./launcher rebuild app at time t1 and discourse works. I then run ./launcher rebuild app at time t2 and something is wrong. How do I get the software back to the previous version? I think I could live with the fact that the build is not repeatable if I could roll back to a known good state. Since the launcher has already built a working docker image at time t1, is it possible to tell the launcher to use a specific image rather than the bad one that was built at time t2?
Any ideas?
Sorry for going a bit off topic on this, I can re-post if you like.