phosphorus: Move docker container info into RunTestRequest.

- Initially, the F20 containers will be used in run-test, not
prejob. The container will be pulled in run-test if needed
(i.e. if it has not already been pulled on the host). In the
future, a mechanism to ensure the same container is used by
prejob and run-test can be used if needed.

BUG=b:201431966
TEST=CQ

Change-Id: I656ae4f47aca4a4974f140054a618fe0c35716f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/3229180
Reviewed-by: Sean McAllister <smcallis@google.com>
Commit-Queue: Andrew Lamb <andrewlamb@chromium.org>
5 files changed