docker containerizer should override entrypoint when starting executor.

When I specify an ExecutorInfo with a ContainerInfo set, the docker containerizer will do docker run ... /bin/sh -c ./my-executor.

If the docker image the user specifies has an entrypoint configured the executor command will be appended to the entrypoint rather than overwriting it. The docker docs says:

Unlike the behavior of the CMD instruction, The ENTRYPOINT instruction adds an entry command that will not be overwritten when arguments are passed to docker run. This allows arguments to be passed to the entry point, i.e. docker run <image> -d will pass the -d argument to the entry point.

This results in docker running the entrypoint inside the container with the executorinfo's command as an argument. If you specified an executor this is never what you want.

I suggest docker run use --entrypoint when starting an executor inside the container.