add sourcegraph/alpine base image; use static GID/UID

Created by: slimsag

This change:

1. Publishes a new sourcegraph/alpine base Docker image, which contains a few defaults (like creating a sourcegraph user/group).
2. Makes all of our Docker images utilize it.
3. Makes our base sourcegraph/alpine image create the sourcegraph user/group with a static ID.

Aside from the 3rd point, there is no functionality change in this PR. It is simply moving what we did previously into a separate Docker base image, and documenting why we do certain things (like installing bind-tools), so this is a very non-risky change despite it looking substantial at first glance.

Fixes #1996 Fixes #1884 Created #2299 Created #2302