Docker: get hash from image name

Particularly if you are using a static image name like “latest” or “develop”, it is useful to be able to know and verify the exact version via the sha256 hash. To get the value you can use:

docker inspect --format='{{index .RepoDigests 0}}' repo/image-name:tag

To use this in a CI/CD workflow for Docker Swarm, you can do something like this:

SHA=$(docker inspect --format='{{index .RepoDigests 0}}' repo/image-name:tag)
docker service update --image $SHA --detach=true --with-registry-auth service-name

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