migrate off docker hub autobuild

Right now the deploy and example yaml files point to docker hub images built via their autobuild service, but Docker is turning off that service on June 18th, 2021, so those images won't be updated after that. Need to find an alternative to build these images:
```
mpioperator/mpi-operator
mpioperator/tensorflow-benchmarks
mpioperator/kubectl-delivery
mpioperator/mxnet-horovod
```
@terrytangyuan

Start by locating the deploy and example YAML files that reference the four mpioperator images, then review how those images were built through Docker Hub autobuild. The work is done when an alternative build process keeps all four images updated after Docker Hub autobuild is unavailable.

Written by the indexing model from the issue text.