`yarn install` failing out of the blue

I have a standard Dockerfile, it is basically the same one generated by flyctl with some small changes…
since this afternoon, the deploy is failing with this error:

 => ERROR [node_modules 1/1] RUN yarn install                                                                                                                                                                     0.8s
 => CANCELED [gems 1/5] RUN gem install -N bundler -v 2.3.23                                                                                                                                                      0.8s
 > [node_modules 1/1] RUN yarn install:
#18 0.732 Usage Error: No project found in /app
#18 0.732
#18 0.732 $ yarn install [--json] [--immutable] [--immutable-cache] [--check-cache] [--inline-builds] [--mode #0]
Error failed to fetch an image or build from source: error building: executor failed running [/bin/bash -c yarn install]: exit code: 1

I have no idea what is going on since I haven’t even done anything on this app today

has anyone seen something like this before? am I missing something really trivial?

this is my dockerfile

update: it works when I stop doing multistage builds, but, this is weird…