misc(docker) add alpine-based Dockerfile

[abreksa4]

Using the python:3-alpine image drops the final image size to 103MB, instead of the 946MB of the python:3 based image.

[adrienverge]

OK to use the alpine-based image, however I prefer not having/maintaining the Dockerfile (or any package source file) in the repository. Can you update the snippet in README.rst instead?