docker-compose.yml file says:
volumes: - /home/deploy/apps/myapp:/go/src/example.com/myapp
which means your host system's source directory is mounted over, and hides, everything that the Dockerfile builds.
./myapp is the host's copy of the
myapp executable and if something is different (maybe you have a MacOS or Windows host) that will cause this error.
This is a popular setup for interpreted languages where developers want to run their application without running a normal test-build-deploy sequence, but it doesn't really make sense for a compiled language like Go where you don't have a choice. I'd delete this block entirely.