fix: integrate PR #3315 review feedback

- ci: add Docker Hub login step, add HF_USER_TOKEN guard on eval step
- docker: tie patches to pinned versions with removal guidance, remove
  unnecessary HF_TOKEN for public dataset, fix hadolint warnings
- docs: fix paper link to arxiv, add teaser image, fix camera names
  (4→3 cameras), fix observation dims (480x640→240x320)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker/Dockerfile.benchmark.robotwin

@@ -60,8 +60,11 @@ RUN cd ${ROBOTWIN_ROOT}/envs \
        uv pip install -e . --no-build-isolation --no-cache
 
 # Upstream patches (mirror RoboTwin's script/_install.sh).
-# mplib: drop a broken `or collide` clause in planner.py.
-# sapien: fix URDF loader encoding + .srdf extension check.
+# These patches target the exact versions pinned above; re-check when upgrading.
+# mplib==0.2.1: drop a broken `or collide` clause in planner.py.
+#   Safe to remove once mplib > 0.2.1 ships with the fix upstream.
+# sapien==3.0.0b1: fix URDF loader encoding + .srdf extension check.
+#   Safe to remove once sapien > 3.0.0b1 ships with the fix upstream.
 RUN python - <<'EOF'
 import pathlib, re, site
 for d in site.getsitepackages():
@@ -80,7 +83,7 @@ EOF
 
 # Simulation assets from TianxingChen/RoboTwin2.0: embodiments (~220 MB) +
 # objects (~3.74 GB). background_texture (~11 GB) is intentionally skipped.
-ARG HF_TOKEN=""
+# The dataset is public — no auth token needed.
 RUN python - <<'EOF'
 import os, pathlib, zipfile
 from huggingface_hub import hf_hub_download
@@ -92,7 +95,6 @@ for fname in ("embodiments.zip", "objects.zip"):
         repo_id="TianxingChen/RoboTwin2.0",
         repo_type="dataset",
         filename=fname,
-        token=os.environ.get("HF_TOKEN") or None,
         local_dir=str(assets_dir),
     )
     with zipfile.ZipFile(local, "r") as z:
@@ -100,10 +102,14 @@ for fname in ("embodiments.zip", "objects.zip"):
     pathlib.Path(local).unlink()
 EOF
 
-RUN cd ${ROBOTWIN_ROOT} && python script/update_embodiment_config_path.py
+WORKDIR ${ROBOTWIN_ROOT}
+RUN python script/update_embodiment_config_path.py
 
 ENV PYTHONPATH="${ROBOTWIN_ROOT}:${PYTHONPATH}"
 
+# Return to the lerobot source directory (set by base image) before overlaying.
+WORKDIR /lerobot
+
 # Overlay the PR's source code on top of the nightly image.
 COPY --chown=user_lerobot:user_lerobot . .