Fix OpenEnv docker-image example bootstrap

[sergiopaniego]

What does this PR do?

Fixes the --env-mode docker-image bootstrap in the OpenEnv catch.py and sudoku.py examples, broken by two OpenEnv changes:

• from_docker_image() is async-first, so calling it synchronously returned a coroutine (auto-detection in OpenEnv#874 covers reset()/step() but not the classmethod constructors, as noted by @burtenshaw in Fix OpenEnv examples broken by async-first client API #6194).
• The public EnvClient.base_url property was removed (added in OpenEnv 0041641c, gone after the core refactor), so _bootstrap.base_url raised AttributeError.

The docker-image bootstrap only needs the container URL, so it now uses LocalDockerProvider().start_container() (the provider exposes a public base_url), mirroring OpenEnv's own examples/browsergym_harness_eval_common.py. Verified end-to-end against a local textarena-env container.

Also drops the docker-hub mode: it relied on Env.from_hub(), which no longer exists in OpenEnv.

Follow-up to #6194 (the .sync() PR, superseded by OpenEnv 0.4.0) and #6138.

Before submitting

• Did you read the contributor guideline, Pull Request section?
• Was this discussed/approved via a GitHub issue? Discussed in Fix OpenEnv examples broken by async-first client API #6194.

AI writing disclosure

• AI-assisted: some parts were suggested or improved by AI, but the PR was written and reviewed by a human.

Who can review?

@burtenshaw @qgallouedec

---

Note

Low Risk
Example-only CLI and bootstrap changes; no training library or auth paths affected.

Overview
Repairs --env-mode docker-image in the OpenEnv catch.py and sudoku.py GRPO examples so they work with current OpenEnv instead of broken from_docker_image() / base_url usage.

docker-image now starts the env with LocalDockerProvider: start_container(args.env_image) for the URL, then wait_for_ready(env_url), matching OpenEnv’s recommended container bootstrap.

docker-hub is removed from CLI choices and setup branches because Env.from_hub() is no longer available.

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[bot-ci-comment]

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