I came here to say the same thing. Images and their layers are loosely related to the Dockerfile format (each command is a layer), but Dockerfiles are as useful

I came here to say the same thing. Images and their layers are loosely related to the Dockerfile format (each command is a layer), but Dockerfiles are as useful for execution as C Source files are. Likewise images can be sent and received without a Dockerfile just like an executable without its source.