tes-runner should retry Docker image notfound

[MattMcL4475]

Problem:
It seems that GCR returns "notfound" when rate-limiting, which causes the task to fail: Docker.DotNet.DockerImageNotFoundException: Docker API responded with status code=NotFound, response={"message":"No such image: us.gcr.io/broad-gatk/gatk@sha256:53b4590c3c8d2d5db7a02c07754a69ea134841f6a08a5a16b238bbf2d6ebd250"}

Solution:
Retry, but add a note in the comments that this is a workaround for a suspected GCR bug and should be removed later if confirmed