sg: add Dockerfile and build script
Created by: jhchabran
This creates a us.gcr.io/sourcegraph-dev/sg
image that is available for embedding sg
in other images. Added docs as well.
Fixes https://github.com/sourcegraph/sourcegraph/issues/31983
The image is built on CI at the same time as the others.
Test plan
- Built locally and checked that
sg
is available from within the Docker image. - CI