Add Docker-native container image

[Marrrrrrrrrrtin]

Summary

Adds a Docker-native build path for the standard PostTrainBench environment:

• containers/docker/standard.Dockerfile
• containers/docker/README.md
• a short README pointer from the Quick Start section

This is intended for cloud GPU environments that already run workloads inside
Docker containers and do not expose Apptainer/Singularity to users. Runpod Pods
are one example, but the Dockerfile is not Runpod-specific.

The existing Apptainer/HTCondor path remains unchanged.

Notes

• The Docker image installs the same main Python/CLI stack as
  containers/standard.def.
• The Dockerfile uses a Python 3.11 virtual environment under
  /opt/posttrainbench-venv, because current inspect_evals releases require
  Python 3.11 or newer.
• SSH is intentionally not included in the base image. Providers that need SSH
  access can add that in a downstream image or provider template.

Validation

• git diff --check
• scanned the changed files for private/local strings before pushing
• started docker buildx build --check -f containers/docker/standard.Dockerfile .;
  it parsed the Dockerfile and then waited on Docker Hub base-image metadata, so
  I canceled the check before any full image build
• separately validated an equivalent Docker-native image on a cloud H100 pod:
  CUDA/H100 visible, torch/vllm/inspect_ai importable, and a small
  Qwen3-1.7B GSM8K smoke run reached GPU use

This PR does not claim a full official benchmark run.