Base image pull in Dockerfile fails with 403

Attempting to use a private base image as the FROM line in a Build Trigger results in a 403 error.

Robot account credentials are required

In order to use a private base image as the FROM line in a Build Trigger, credentials for a Robot Account with read access to the private image must be specified when setting up the build trigger.

Specifying robot account credentials

A robot account to use to pull the base image can be chosen in the build trigger setup page before the trigger is created: