use sourcegraph/alpine even when building

Created by: slimsag

By not using our sourcegraph/alpine base image and instead using alpine during building phases, we have a few risks:

1. Confusion: "Why do we use alpine here but sourcegraph/alpine here?"
2. Nuanced dependency issues: "it works fine in the build stage but then fails when running, there is a dependency version difference between the build stage and sourcegraph/alpine due to version differences?!"
3. Confusion: "why does that PR not bump alpine for the searcher Dockerfile? Can I just add it manually (in a separate PR, or in that renovate PR?)"
4. Not keeping sourcegraph/alpine up-to-date: someone chose to use alpine for e.g. tracking-issue bot instead of sourcegraph/alpine, I don't know why.
5. Introducing new root services, which is very bad for security and painful to migrate away from.

We should just use sourcegraph/alpine everywhere

(this has zero effect on which user the containers run as, but did help to uncover https://github.com/sourcegraph/sourcegraph/issues/13237 )

Helps #13035 (closed)