Add docker file for spark connect server.

[imvtsl]

We can reduce effort in Step 4 of README.md if we run Spark Connect server in a docker container.

Doing this would also help with this step when setting up a spark-connect-go application.

Proposed solution:

Have a DockerFile or a docker compose file that runs Spark Connect Server on port 15002. Then, we could simply do docker-compose up to get the spark connect server ready.

I can raise a PR with this proposed solution.

[imvtsl]

Hey @grundprinzip
Do we want to pursue this? If yes, I can raise a PR.