fix(docker): reject unsupported --user <arbitrary-uid> start with clear guidance (#38579)

`docker run --user $(id -u):$(id -g)` was a tini-era trick to make
container-written files match the host user. Under s6-overlay it no longer
works: the bootstrap (UID remap, volume + build-tree chown, config seeding)
needs root, and the baked image dirs (/opt/data, /opt/hermes/.venv, ui-tui,
node_modules) are owned by the hermes build UID (10000). A pinned arbitrary
UID can't write them, so the runtime fails with EACCES on a bind mount or
hard-crashes on a named volume (Docker inits the volume from the image as
10000; the non-root start can't even `cd /opt/data`, and the profile
reconciler dies with PermissionError on gateway_state.json).

Detect that start early in both the cont-init hook (stage2-hook.sh) and the
CMD wrapper (main-wrapper.sh) and fail fast with actionable guidance pointing
at the supported path: root start + HERMES_UID/HERMES_GID (or the PUID/PGID
aliases), which remaps the hermes user and chowns the volume — the same
host-UID-matching outcome --user was used for, without breaking s6.

The guard fires only when the current UID is neither root NOR the hermes UID.
This preserves the supported non-root start from #34648/#34837 (running with
`--user 10000:10000`, i.e. pinned to the hermes UID itself), which is
unaffected — only the arbitrary-UID variant that #34837 never actually made
writable is rejected.

Verified live across five scenarios (built image, bind + named volume):
arbitrary --user on bind -> rejected with guidance, hermes does not run;
arbitrary --user on named volume -> guidance shown, no raw 'can't cd' crash;
--user 10000:10000 -> boots; root + HERMES_UID=4242 remap -> boots, guard not
tripped; default root start -> boots. Pre-fix control reproduces the raw
PermissionError + 'can't cd' crash with no guidance.

tests/tools/test_stage2_hook_user_flag_guard.py

@@ -0,0 +1,119 @@
+"""Contract test: the s6-overlay stage2 hook and main-wrapper reject an
+unsupported `docker run --user <arbitrary-uid>:<gid>` start with actionable
+guidance, while still allowing:
+
+  - root start (id -u == 0)
+  - `--user <hermes-uid>` (the supported non-root start, #34648 / #34837)
+
+Background: in the tini era `docker run --user $(id -u):$(id -g)` was used to
+make container-written files match the host user. Under s6-overlay this can't
+work — the bootstrap (UID remap, volume/build-tree chown, config seeding) needs
+root, and the baked image dirs are owned by the hermes build UID, so an
+arbitrary pinned UID can't write them (EACCES on a bind mount, hard crash on a
+named volume). The supported path is root start + HERMES_UID/HERMES_GID (or the
+PUID/PGID aliases), which remaps the hermes user and chowns the volume.
+
+The guard fires only when the current UID is neither root NOR the hermes UID,
+so the #34648 `--user 10000:10000` case (pinning to the hermes UID itself) is
+unaffected.
+
+Extraction + stubbed-shell-run mirrors
+tests/tools/test_stage2_hook_toplevel_chown.py.
+"""
+from __future__ import annotations
+
+import re
+import shutil
+import subprocess
+import tempfile
+from pathlib import Path
+
+import pytest
+
+REPO_ROOT = Path(__file__).resolve().parents[2]
+STAGE2_HOOK = REPO_ROOT / "docker" / "stage2-hook.sh"
+MAIN_WRAPPER = REPO_ROOT / "docker" / "main-wrapper.sh"
+
+
+def _read(p: Path) -> str:
+    if not p.exists():
+        pytest.skip(f"{p} not present in this checkout")
+    return p.read_text()
+
+
+def _guard_block(text: str) -> str:
+    """Extract the `cur_uid=...; if [ ... ]; then ... exit 1; fi` guard."""
+    m = re.search(
+        r"(cur_uid=\"\$\(id -u\)\"\nif \[ \"\$cur_uid\" != 0 \](?:.*\n)*?fi)",
+        text,
+    )
+    assert m, "expected the --user guard block (cur_uid + non-root/non-hermes check)"
+    return m.group(1)
+
+
+@pytest.mark.parametrize("path", [STAGE2_HOOK, MAIN_WRAPPER])
+def test_guard_present_and_mentions_remediation(path: Path) -> None:
+    text = _read(path)
+    block = _guard_block(text)
+    # Must check non-root AND non-hermes-uid (so --user 10000:10000 is allowed).
+    assert '"$cur_uid" != 0' in block
+    assert '"$cur_uid" != "$(id -u hermes)"' in block
+    assert "exit 1" in block
+    # Must point users at the supported env vars.
+    assert "HERMES_UID" in block and "HERMES_GID" in block
+    assert "PUID" in block and "PGID" in block
+
+
+def _run_guard(text: str, *, cur_uid: int, hermes_uid: int = 10000) -> subprocess.CompletedProcess:
+    """Run the extracted guard with `id` stubbed. Returns the completed process
+    (rc 1 + stderr message when rejected, rc 0 when allowed through)."""
+    bash = shutil.which("bash")
+    if bash is None:
+        pytest.skip("bash not available")
+    block = _guard_block(text)
+    with tempfile.TemporaryDirectory() as d:
+        script = (
+            "set -e\n"
+            # Stub `id`: `id -u` -> cur_uid; `id -u hermes` -> hermes_uid.
+            f'id() {{ if [ "$2" = hermes ]; then echo {hermes_uid}; else echo {cur_uid}; fi; }}\n'
+            + block
+            + "\necho GUARD_PASSED\n"  # only reached when the guard allows through
+        )
+        sp = Path(d) / "h.sh"
+        sp.write_text(script)
+        return subprocess.run([bash, str(sp)], capture_output=True, text=True)
+
+
+def test_arbitrary_user_uid_is_rejected() -> None:
+    """An arbitrary host UID (1000), neither root nor hermes, is rejected."""
+    for text in (_read(STAGE2_HOOK), _read(MAIN_WRAPPER)):
+        proc = _run_guard(text, cur_uid=1000, hermes_uid=10000)
+        assert proc.returncode == 1, f"expected rejection, got rc={proc.returncode}"
+        assert "not supported" in proc.stderr
+        assert "GUARD_PASSED" not in proc.stdout
+
+
+def test_root_start_passes() -> None:
+    """Root start (uid 0) is never blocked."""
+    for text in (_read(STAGE2_HOOK), _read(MAIN_WRAPPER)):
+        proc = _run_guard(text, cur_uid=0, hermes_uid=10000)
+        assert proc.returncode == 0, proc.stderr
+        assert "GUARD_PASSED" in proc.stdout
+
+
+def test_user_pinned_to_hermes_uid_passes() -> None:
+    """`--user 10000:10000` (the hermes UID itself) is the supported non-root
+    start from #34648 / #34837 and must NOT be blocked."""
+    for text in (_read(STAGE2_HOOK), _read(MAIN_WRAPPER)):
+        proc = _run_guard(text, cur_uid=10000, hermes_uid=10000)
+        assert proc.returncode == 0, proc.stderr
+        assert "GUARD_PASSED" in proc.stdout
+
+
+def test_user_pinned_to_remapped_hermes_uid_passes() -> None:
+    """After a HERMES_UID remap the hermes UID is e.g. 4242; a container pinned
+    to that same UID must still pass (cur_uid == hermes_uid)."""
+    for text in (_read(STAGE2_HOOK), _read(MAIN_WRAPPER)):
+        proc = _run_guard(text, cur_uid=4242, hermes_uid=4242)
+        assert proc.returncode == 0, proc.stderr
+        assert "GUARD_PASSED" in proc.stdout