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