Make Docker image smaller (1.21GB → 558MB)

[tiulpin]

What these changes are about:

• switch from python:3.6 to python:3.6-slim-buster
• install libgomp1 (it's missed in slim-buster images)
• clean apt caches even more than before

The result: Docker image size reduced from 1.21GB to 558MB (local, non-compressed size).
Didn't find the tests for Docker images, but checked: everything is working as before, but the image is smaller, easier to download from Docker Hub, and use!

Tasks

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[pombredanne]

Thank you ++
I have a minor comment inline.

Eventually we should get that image built and scanned with https://github.com/nexB/scancode.io/blob/main/scanpipe/pipelines/docker.py ;)

[pombredanne]

@tiulpin you wrote:

Didn't find the tests for Docker images, but checked: everything is working as before, but the image is smaller, easier to download from Docker Hub, and use!

there is no tests yet... if you know how to add some that would be sweet-o-sweet!

Unrelated, since you seem to dabble in ML, you may be interested to check out @AyanSinhaMahapatra WIP on this scancode plugin https://github.com/nexB/scancode-results-analyzer/ that tries to use some ML to spot (and fix) inconsistent license detections.

[tiulpin]

there is no tests yet... if you know how to add some that would be sweet-o-sweet!

Will try to add it in next PRs, OK?)

But now this is ready to be merged for sure, including your resolved comment.

[pombredanne]

LGTM, merging!
Thank you ++