CI: set concurrency limit of 1 on Docker steps
Created by: chrismwendt
Previously, Docker builds could starve the sourcegraph/server container that was the subject of e2e tests, causing initialization to take >30s and timeout, causing build errors.
This PR applies the concurrency limit of 1 to all Docker steps, not just the e2e runs.
This will slow down CI, but there are some other ways to mitigate that: https://github.com/sourcegraph/sourcegraph/issues/2657#issuecomment-472219579