`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.
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