Allow configuring Duo Workflow Remote Execution with a custom docker image

Problem

Today Duo Workflow Remote Execution is not really suitable for doing any software development tasks as it doesn't have any developer tools installed. You can read more at #548239 (closed) .

Solution

We can take a very small incremental step here to improve the usefulness of Remote Execution to add a field to the API for creating workflows to specify an image. This image could be saved in the database and used later when creating the workload to set the image used in the CI job.