reduce image size. Fixes #741

[lfservin]

This is a fix for #741 The recommendations from semgrep with dockerfile ruleset have been followed:

• do not install recommends,
• call apt-get clean and clean the cache,
• finally use --no-cache-dir with pip

Additionally, changed the image base from python:3.7 to python:3.7-slim

The resulting image is around 800MB vs 2.2 GB that the original had

[blabla1337]

Thanks for the pull request, much appreciated :) This is really cool stuff, actually the original was around 900MB and now with those suggested improvements to 300MB https://hub.docker.com/layers/blabla1337/skf-api/dev/images/sha256-4be3f360f37812428e977003740b0bf0f07a2f108d974c1c49d7e64bf77dbc0a?context=repo

Again thanks for the pull request, will merge and build a new image next week!