For more information about this image and its history, please see the relevant manifest file (library/pypy) in the docker-library/official-images GitHub repo.
PyPy started out as a Python interpreter written in the Python language itself. Current PyPy versions are translated from RPython to C code and compiled. The PyPy JIT (short for "Just In Time") compiler is capable of turning Python code into machine code at run time.
These images include multiple ONBUILD triggers, which should be all you need to bootstrap most applications. The build will COPY a requirements.txt file,RUN pip install on said file, and then copy the current directory into/usr/src/app.
This image makes building derivative images easier. For most use cases, creating a Dockerfile in the base of your project directory with the line FROM pypy:onbuild will be enough to create a stand-alone image for your project.