This plugin automatically downloads the alpine image to perform various service tasks. Pulling the alpine image fails, when using a custom registry for the build image with credentials.

Setup:

• Make sure the required alpine image (3.6 as of time of writing this report) is not available locally on the docker host.
• Use Image ID referring to custom registry. For example: registry.mycompany.com/environment/build:latest 
• Select appropriate credentials for registry.mycompany.com from the dropdown menu
• Select verbose to see the error messages

Result:
Build fails before the actual build image is started, with the following error messages:

docker run --rm --entrypoint /bin/true alpine:3.6
Unable to find image 'alpine:3.6' locally
docker: Error response from daemon: Get https://registry-1.docker.io/v2/library/alpine/manifests/3.6: unauthorized: incorrect username or password.
See 'docker run --help'.
FATAL: Failed to run docker image registry.mycompany.com/environment/build:latest

Please note that the last message is especially misleading as the problem does not stem from any issues with the build image.

Probable cause:
docker run attempts to use the credentials provided for the custom registry to authenticate with the public docker registry.