Use smaller docker base image

[john-theo]

python:3.7 = 918M
python:3.7-slim = 179M

But uWSGI installation seems broken on python:3.7-slim, some tweaks still needed. Will be looking into it after the whole misago system runs properly on my machine.

Also, add custom sources for apt and pip may be a good idea : )

[john-theo]

Sorry, didn't read the "Need help" section carefully, moving this to the forum and closing the issue...

[rafalp]

This is actualy valid thing to keep on our roadmap :)

[rafalp]

Ideally Misago could have single Dockerfile that would build python and JS dependencies in full base image, then do smaller image that would only copy over final python (and in future node.js) envs.

This image would be used by misago and celery services (which currently built same image twice).