GH #4290
Add digest support to the docker driver image config. This commit
factors out some common code to print the repo:tag (dockerImageRef) for
events/logs as well as parsing the image to retreive the repo,tag
(parseDockerImage) so that the results are consistent/sane for both
repo:tag and repo@sha256:... references.
When pulling an image with a digest, the tag is blank and the repo
contains the digest. See:
https://github.com/fsouza/go-dockerclient/blob/master/image_test.go#L471
If two jobs are pulling the same image simultaneously, which ever starts the pull first will set the pull timeout.
This can lead to a poor UX where the first job requested a short timeout while the second job requested a longer timeout
causing the pull to potentially timeout much sooner than expected by the second job.
This PR introduces a coordinator for doing CRUD on a Docker image. It
should fix racy deletion of images. The issue before was images would be
deleted between prestart and start causing an error.
