How to update the build image

The build image currently can only be updated by a Cortex maintainer. If you’re not a maintainer you can still open a PR with the changes, asking a maintainer to assist you publishing the updated image. The procedure is:

1. Update build-image/Docker
2. Build the image running make build-image/.uptodate
3. Publish the image to the repository running docker push quay.io/cortexproject/build-image:TAG (this can only be done by a maintainer)
4. Replace the image tag in .circleci/config.yml (there may be multiple references)
5. Open a PR and make sure the CI with new build-image passes