Update README and Dockerfile

[adamreeve]

This PR includes a few tidy ups and fixes to examples in the README:

• Update versions of Arrow and DataFusion referenced in the README
• Update the version of datafusion-java used
• Update the Dockerfile so that jshell works again and update the section about using jshell in the README
• Fix invalid references to ExecutionContexts that should be SessionContexts
• Update the example code to include the full class definition and required import statements to make it easier to use

Some of this work is thanks to @pavlovic-ivan and was originally contributed in the G-Research fork