Update all images to alpine:3.12

Created by: rvantonder

I'd like to update our alpine images to 3.12 to merge more recent version of comby and need some help (context: https://github.com/sourcegraph/sourcegraph/issues/13028#issue-679401406)

I see this renovate PR bumps alpine to 3:12. Questions:

• is it safe to merge? what could break?
• 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?)

CC @beyang since I see you opened related issue https://github.com/sourcegraph/sourcegraph/issues/10161 in the past.