3 minute read  


When transferring data among networked systems, trust is a central concern. In particular, when communicating over an untrusted medium such as the internet, it is critical to ensure the integrity and immutability of all the data a system operates on. Especially if you use Docker Engine to push and pull images (data) to a public registry.

This immutability offers me a guarantee that any and all containers that I instantiate will be absolutely identical at inception. Surprise surprise, deterministic operations.

A Lesson in Deterministic Ops

Docker Tags are about as reliable and disposable as this guy down here.


Seems simple enough. You have probably already deployed hundreds of YAML’s or started endless count of Docker container.

docker run --name mynginx1 -P -d nginx:1.13.9


apiVersion: apps/v1
kind: Deployment
  name: rss-site
  replicas: 1
      app: web
        app: web
        - name: front-end
          image: nginx:1.13.9
            - containerPort: 80

But Tags are mutable and humans are prone to error. Not a good combination. Here we’ll dig into why the use of tags can be dangerous and how to deploy your containers across a pipeline and across environments, you guessed it, with determinism in mind.

I want to ensure that whether it’s today or 5 years from now, that specific deployment uses the very same image that I defined. Any updates or newer versions of an image should be executed as a new deployment. The solution: digest

A digest takes the place of the tag when pulling an image, for example, to pull the above image by digest, run the following command:

docker run --name mynginx1 -P -d nginx@sha256:4771d09578c7c6a65299e110b3ee1c0a2592f5ea2618d23e4ffe7a4cab1ce5de

You can now make sure that the same image is always loaded at every deployment. It doesn’t matter if the TAG of the image has been changed or not. This solves the problem of repeatability.

Content Trust

However, there’s an additionally hidden danger. It is possible for an attacker to replace a server image with another one infected with malware.


Docker Content trust gives you the ability to verify both the integrity and the publisher of all the data received from a registry over any channel.

Prior to version 1.8, Docker didn’t have a way to verify the authenticity of a server image. But in v1.8, a new feature called Docker Content Trust was introduced to automatically sign and verify the signature of a publisher.

So, as soon as a server image is downloaded, it is cross-checked with the signature of the publisher to see if someone tampered with it in any way. This solves the problem of trust.

In addition you should scan all images for known vulnerabilities, this can fill another book