ci: Move Docker labels declaration inside Dockerfile

Created by: ggilmore

Follow up to https://github.com/sourcegraph/sourcegraph/pull/2806

This PR:

1. Moves the COMMIT_SHA, etc. labels to be declared inside the Dockerfile via build args instead of in the docker build ... command. This is slightly cleaner, and makes it so that people don't need to remember the full metadata key (org.opencontainer...) when modifying the build scripts.
2. Adds basic URL, docs metadata labels to the sourcegraph/alpine image. All images that use sourcegraph/alpine as a base will inherit the value of these labels. cc @slimsag (The sourcegraph/alpine image will need to be re-deployed after this change)