When MKB and I were pairing on concourse-tutorial task 12, the following error occurred when we ran the pipeline.

resource script '/opt/resource/check []' failed: exit status 1

stderr:  
failed to fetch digest: 401 Unauthorized  

In the tutorial, there is a bump-timestamp-file.yml which was using the following docker image resource:

image_resource:  
  type: docker-image
  source: { repository: concourse/concourse-ci }

We explored the Concourse Docker Files and found out that the Concourse team had changed concourse/concourse-cli image for tag v1.4.1 to concourse/bosh-cli. Additionally, as of tag v1.5.0, the concourse/concourse-ci image does not exist in Docker Hub anymore. We changed the image_resource configuration to use the available Docker image concourse/bosh-cli as follows, which solved the problem.

image_resource:  
  type: docker-image
  source: { repository: concourse/bosh-cli }

If similar error happens for you in the future, remember to check if your Docker Image exists and is accessible before you pull all of your hair out!