provisioner: support docker_image and pre_start_cmd for Gemma 4 startup

Adds docker_image and pre_start_cmd config fields to config.rb and
provisioning.rb so the Gemma 4 31B workarounds are baked in:
- docker_image = "vllm/vllm-openai:nightly" (stable lacks Gemma 4 support)
- pre_start_cmd = "pip install -q transformers==5.5.0" (stable pins <5)
- extra_docker_env = ["CUDA_VISIBLE_DEVICES=0"] (required with --entrypoint bash)

When pre_start_cmd is set, the provisioner switches to --entrypoint bash and
chains the patch command before launching vLLM, so create-both works end-to-end
without manual container replacement.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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