I was using DC/OS-Marathon and needed to pull image from a private repository on Docker Hub or a custom registry like Azure Container Registry.

TL;DR

Follow this link: https://mesosphere.github.io/marathon/docs/native-docker-private-registry.html, but in step 2 replace the uris section with

  "fetch" : [
    {
      "uri" : "https://path.to/file?including=querystring&parameters=andstuff",
      "extract" : true,
      "outputFile" : "dockerConfig.tar.gz" 
    }
  ]

Deep Dive

The doc mentioned above is split in 2 steps. step 1 is fine: you docker login into your private repo or custom registry. This creates a config file with your credentials under ~/.docker/config.json. You then package that up, and put it in a shared location. In my case I took the package and put it in Azure Blob Storage container. So far so good.

In Step 2 of the doc, you should reference that shared package from your Marathon app configuration. Here I had some issues:

First, the doc instructs you to use uris, which are considered deprecated, and should be replaced with fetch.

Second, I was planning to reference the file using Azure Storage SAS uri. Which means the path to the file will include some query string parameters. That will bite us back in a moment.

If you followed the doc exactly, you archived the credentials file into .tar.gz. That’s not absolutely necessary, but it’s what the docs walks you through. If you did that, wou will have to add extract:true to the fetch section.

The Mesos extraction process checks the file extension and will not extract if the file doesn’t have a known extension. Since I used a SAS uri, the file’s extension was not picked up and Mesos failed to extract it.

A solution to this is to use Fetch’s outputFile field, which will rename the downloaded file before attempting to extract.
This is not consistently documented, but you could add outputFile to the fetch section and it will work fine.

The resulting marathon app definition looks like this:

{
  "id" : "...",
  "container" : {...},
  "fetch" : [
    {
      "uri" : "https://path.to/file?including=querystring&parameters=andstuff",
      "extract" : true,
      "outputFile" : "dockerConfig.tar.gz" 
    }
  ]
}