add sourcegraph/alpine base image; use static GID/UID
Created by: slimsag
This change:
- Publishes a new
sourcegraph/alpine
base Docker image, which contains a few defaults (like creating asourcegraph
user/group). - Makes all of our Docker images utilize it.
- Makes our base
sourcegraph/alpine
image create thesourcegraph
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 (closed) Fixes #1884 (closed) Created #2299 (closed) Created #2302