Caveats: 1) this may not be what grandparent was trying to do and 2) I have limited experience debugging "regular" Docker, let alone firecracker or whatever Fly.io does.
With that said, I succesfully deployed a simple Rust/Actix/Sqlite app with a Dockerfile to Fly.io. I then thought I'd try out litestream. For reasons I'm still not sure about, having `ENTRYPOINT ["litestream replicate … -exec myapp"]` resulted in immediate kernel panics.[1]
As I was debugging, my instinct was to want to `fly ssh console` into a running container to see if running the same command from the shell produced any clues for further debugging. Though I understand this is the wrong mental model, the thought was something like "well, I know everything else works, I just need Fly to ignore this failing command so I can have a minute to poke around." To do this, I ended up just removing litestream from the ENTRYPOINT so the deploy would succeed, then I could SSH in and play around at the shell to see what was going on.
Again, I have no idea whether this is the same sort of problem the other person was having, but for my case, what would be helpful is probably not changing how `fly ssh console` behaves, but perhaps some documentation of suggested debugging techniques in case your app is failing to start.
[1] Putting the exact same command into a "start.sh" and making that the ENTRYPOINT worked fine so that's what I ended up doing.