Run `docker-tools-eng-validation-pr` pipeline as part of PR validation when there are changes to ImageBuilder

We have two PR validation pipelines in this repo.

1. The ImageBuilder build pipeline
2. https://github.com/dotnet/docker-tools/issues/305

[This eng validation pipeline](https://github.com/dotnet/docker-tools/issues/305) runs only when there are changes to the eng/ folder:

https://github.com/dotnet/docker-tools/blob/736b6b0f3ea2923a754a51ec7726074e7fe5ebb7/eng/pipelines/dotnet-docker-tools-eng-validation-pr.yml#L1-L8

However, this does not check when changes in ImageBuilder could affect the way the eng-validation pipeline runs, since it uses the version of ImageBuilder from MAR.

To provide a check for this, we should change the eng-validation pipeline to build and run a new ImageBuilder image, and use that image to run the regular eng-validation-pr pipeline.

One unknown to solve here is how to pass artifacts between stages of public pipeline builds if we don't have access to an ACR. A potential solution to explore is writing the image to the disk and loading it in each successive stage.