The error message specifically says "not a directory" and afaik you can't mount single files, only directories. I at least have never even thought of trying to

The error message specifically says "not a directory" and afaik you can't mount single files, only directories. I at least have never even thought of trying to mount individual files since the bind mounting functionality in Docker seems to always and everywhere have been described in a way that suggests that it's for mounting directories, not individual files.