14198: Use PDH for container_image instead of docker repo+tag

14198: Use PDH for container_image instead of docker repo+tag

Needed to support federated container requests.

This reverts 6ea807b2caf6c934f170b2e4d89c23c4a08ca69c

Based on the commit comment, that change was made to accomodate the
Docker v1 to v2 image format migration (to enable the API server to
select the image with the correct format).  However, the API server
subsequently gained the ability to detect if it needed to substitute a
PDH with a migrated image PDH in commit
a72205728f94f5261b657766e01f5767dc15d4b5 so now we want restore the
original behavior of locally resolving the image PDH and using that in
the container request.

Arvados-DCO-1.1-Signed-off-by: Peter Amstutz <pamstutz@veritasgenetics.com>