The actually upgrade will be performed manually, but this change will be
used to update the docker-compose.yaml file.
If we land this change prior to the upgrade then note the
manage-projects commands will be updated to use the 3.4 image possibly
while gerrit 3.3 is still running. I don't expect this to be a problem
as manage-projects operates via network protocols.
Change-Id: I5775f4518ec48ac984b70820ebd2e645213e702a
9bfacda1ac