Arvados

The diagnostics run a test container to verify that crunch is working. However, it assumes that an alpine image (or a different image specified on the command line, perhaps arvados/jobs:latest) has been loaded. Our install process doesn't load any images, because arvados-cwl-runner is responsible for loading the image that corresponds to the exact version of a-c-r being used, on demand.

Running the "/hello" command with the "hello-world" image would work just as well, and that image is only ~20KB.

The diagnostics command should embed the hello-world image, and upload it to Keep at runtime, instead of relying on the docker daemon being installed/working on the client host.

Continue to support the -docker-image=... option as well (assume the image is already uploaded).

Requested command should be /hello if using the hello-world image, otherwise echo {timestamp}.