Improve docker image size by using multi-stage build

[NicolasCARPi]

Currently the image is 1.3 GB, this change will bring it down to 972 MB, saving bandwidth and disk space to all.

The venv is built on an image and copied and used on another. This saves us from installing gcc and git in the final image.

Some env vars were added:

• DEBIAN_FRONTEND=noninteractive as is usual in apt scripts
• PYTHONDONTWRITEBYTECODE because it's useless in a container
• PYTHONUNBUFFERED don't try to buffer output
• PIP_DISABLE_PIP_VERSION_CHECK not sure if useful here but doesn't hurt

Note: I've tested it, it works fine as far as I can tell.