restart_process: make compatible with k8s_custom_deploy

[landism]

Problem

#391

The restart_process extension works by setting the docker_build entrypoint arg to point at its restart wrapper script. However, the way k8s_custom_deploy works, docker_build's entrypoint is ignored, which means anyone using restart_process with k8s_custom_deploy would find their processes would not be restarted on update.

Solution

When making the child image for restart_process, also set the docker image's entrypoint itself to the restart wrapper script.
This means k8s_custom_deploy will now work with restart_process unless k8s_custom_deploy returns yaml that sets ContainerSpec.Command. In that case, the process will silently fail to restart and not provide the user any hints.

We still set docker_build's entrypoint. This is sometimes redundant, but needed so that the k8s_yaml that sets ContainerSpec.Command continues to work with this extension.