CI docker push tasks wait for all docker build tasks
The next CI bottleneck...
Commit 51fa5d8c allowed the portable source CI task to run as soon as its Docker image is ready. However, the docker_push_* tasks all have no
needs clauses, resulting in them blocking until all the docker_build_* tasks are complete. So the Ubuntu 18.04 image can be built and ready to be pushed, but its push task won't run until other Docker images have finished building.
needs clauses could break this dependency and increase our build parallelism.