Hello,
since a while the Concourse docker-image resource has an alternative, the drop-in replacement registry-image resource. Development on docker-image has stopped.
The Concourse documentation unfortunately is partially out of date, but to show that registry-image is the preferred type to use I can quote the Concourse introduction documentation:
Everything in Concourse runs in a container. Instead of modifying workers to install build tools, Tasks describe their own container image (typically using Docker images via the registry-image resource).
This snippet configuration:
image_resource:
type: registry-image
source:
repository: alpine
tag: latest
reports the error: The 'registry-type' Resource Type does not exist.

Thanks!