Alpine Linux Docker image

[jpnurmi]

The Alpine Linux Docker image is bare bones (< 5MB!) and requires quite a bit of boilerplate to make it usable for sentry-native's CI workflows. Consider preparing an image that has Bash, Git, CMake, Python etc. pre-installed to reduce the noise in ci.yml.

Suppose the container setup (essentially, now parallel conditional build-steps in the workflow yaml) gets out of hand at some point. In that case, we can also evaluate switching to Dockerfiles written on top of the Alpine image, manually creating images that already contain what we want, and executing the pytest there.

Originally posted by @supervacuus in #1233 (comment)