[Docker][RunPod] Support docker image w/ customized entrypoint on RunPod

[cblmemo]

The fix in #3867 does not work for RunPod since it used the cloud API to directly use docker as the base image, instead of the docker provisioner we used. RunPod also does not provide API for adding any customized arguments to the docker run command. We need to figure out a way to support it on RunPod.

[cblmemo]

Submitted a feature request to RunPod team: runpod/runpod-python#338