Update Dockerfile and Docker README

[IanHoang]

Description

Users on Dockerhub were having trouble running the command docker run opensearchproject/opensearch-benchmark in the README and Dockerhub repo's description. The commands exits with an exit status code of 0 because it doesn't run any entrypoint commands. This PR does the following:

• Adds entrypoint commands to Dockerfile
• Adds more information to the README to illuminate how to use the Docker image.
• Adds the command that's been updated in the Dockerhub repository so that there's consistency between the README and the Dockerhub repository overview tab.

Issues Resolved

Testing

• New functionality includes testing

Built image locally after updating Dockerfile and tested it with the following commands with Docker CLI. Tested on Docker Desktop to confirm consistency:

• docker run --entrypoint bash <IMAGE> -c "opensearch-benchmark -h" --> overwrites entrypoint commands to use opensearch-benchmark help command
• docker run <IMAGE> opensearch-benchmark -h --> runs it in simpler method
• docker run -it <IMAGE> /bin/sh --> runs in interactive mode in terminal
• Ran with Docker Desktop to run the image
• Monitored container status on Docker desktop

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.