interop: README instructions to run locally should use Docker images

The point of using Docker images is that you don't need to set up a local dev environment in that particular language. It's also closer to the setup that's running in CI, which makes it more useful.

Locate the README instructions for running the interoperability tests locally and review how each language-specific environment is set up. Update the instructions to use Docker images, then verify that the documented commands work without requiring a local language environment and remain consistent with CI.

Written by the indexing model from the issue text.