Docker Registry - Do not allow pushing if the tag already exits (except for :latest tag)

Question

I have a scenario where we need our private Docker Registry (v2) to not accept pushes to a tag if it already exists.

For example I have

192.168.0.77:5000/my-project:1.0.0

and someone pushes an update on the endpoint above. It should stop the push.

Then when the user pushes with tag 1.0.1 or any other, it will push successfuly.

I know Docker allows pushing on the same tag, however I wish to have this kind of workflow so we don't override each other's image this way and also these will co-relate with a Jenkins build (for transaction purposes).

Deployment Instructions (in bash)

 docker login -u admin -p fakepassword 192.168.0.77:5000
 docker tag my-project 192.168.0.77:5000/my-project:1.0.0
 docker push 192.168.0.77:5000/my-project:1.0.0

Can someone please advice a way of achieving this?

Answer 1

This is what I use in my CI pipeline.

Check the value of $? , which contains the result of the most recent command - in your case a command that checks if the tag already exists:

#!/bin/bash

docker manifest inspect $IMGNAME:$IMGTAG
RESULT=$?
if [ $RESULT == 0 ]; then
  echo success
else
  echo failed
fi

Save it as a file and call it script.sh

To run the script: sh ./script.sh

The script will return 'success' if the command is successful otherwise it will return 'failed'

Answer 2

The term you're looking for is immutable tags or tag locking. This is a feature of registry servers. If you trust the tooling pushing to the registry, then you can check for an existing tag with various tools:

• docker manifest inspect (I'm not certain if this is always a remote pull, treat this as experimental)
• docker buildx imagetools inspect (this command is hidden, which may mean it will change in the future)
• crane (from Google's go-containerregistry)
• regctl image digest (from myself, the digest does a HEAD request which is faster and better for registries that rate limit requests)
• skopeo (from RedHat)

With each of these, you're looking for error conditions that trigger when the tag is missing and don't care about the output:

if regctl image digest ${some_image} >/dev/null 2>&1; then
  echo image exists, skip push
else
  echo image missing, push new tag
fi