Executor should report the ID of the Docker image being used
Description
I frequently update the Docker images which my builds depend on. Therefore, my builds use a tag (e.g., :master or :experimental) to identify the Docker image. If I update the image, retrigger the build, and the build subsequently fails, I need to know which image was used - but all I can see in the Gitlab CI build console log is the specified Docker image name, including the tag. This does not uniquely identify an image, so I can't be sure if the build used my new image or not.
Proposal
It would be great if the first few lines printed by the runner included the Docker image ID, e.g., like this:
Running with gitlab-ci-multi-runner 1.10.4 (b32125f)
Using Docker executor with image git-registry.example.org/images/my-dev:work-around-problems ...
Pulling docker image git-registry.example.org/images/my-dev:work-around-problems ...
Using docker image 743184ba5... <<<<--- PROPOSED NEW LINE
Running on runner-7a422c71-project-83-concurrent-0 via gitrun2...
Links to related issues and merge requests / references
I'm not aware of any related issues.
The relevant code seems to be https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/blob/master/executors/docker/executor_docker.go#L119