I have a Docker image that was created roughly a year ago. The Dockerfile contains:
FROM docker:stable
How can I determine the actual version of the docker image that stable was referring to back when the image was built?
Edit: What I want to do, in a nutshell, is replace FROM docker:stable with FROM docker:X.Y.Z where X.Y.Z is the version tag that "stable" was pointing to a year ago when the image was originally built.
As suggested by this answer
docker inspect --format='{{index .RepoDigests 0}}' $IMAGE
This will give you the sha256 hash of the image.
Then you can use a service like MicroBadger to get more info about that specific build.
If you want to recreate the Dockerfile you can use docker history to examine the layer history:
$ docker history docker
IMAGE CREATED CREATED BY SIZE COMMENT
3e23a5875458 8 days ago /bin/sh -c #(nop) ENV LC_ALL=C.UTF-8 0 B
8578938dd170 8 days ago /bin/sh -c dpkg-reconfigure locales && loc 1.245 MB
be51b77efb42 8 days ago /bin/sh -c apt-get update && apt-get install 338.3 MB
4b137612be55 6 weeks ago /bin/sh -c #(nop) ADD jessie.tar.xz in / 121 MB
750d58736b4b 6 weeks ago /bin/sh -c #(nop) MAINTAINER Tianon Gravi <ad 0 B
511136ea3c5a 9 months ago 0 B
Keep in mind that if the image has been manually tampered with, I don't know how reliable this output would be.
Finally if you want to go full hacker mode, this old thread on the Docker community forums has some info.
I'm not sure how you can get the tag, because I don't believe this is stored in the image itself, but in the repository. So you'd have to query the repository itself, or get a full list of image history and go detective on it.