Fix sx_docs Dockerfile: install dune + set PATH for OCaml build

The opam base image has dune in the switch but not on PATH.
RUN eval $(opam env) doesn't persist across layers. Install dune
explicitly and set PATH so dune is available in build steps.

Also fix run-tests.sh to respect QUICK env var from caller
(was being overwritten to false).

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

sx/Dockerfile

@@ -3,12 +3,14 @@
 # --- Stage 1: Build OCaml SX kernel ---
 FROM ocaml/opam:debian-12-ocaml-5.2 AS ocaml-build
 USER opam
+RUN opam install dune -y
+ENV PATH="/home/opam/.opam/5.2/bin:${PATH}"
 WORKDIR /home/opam/sx
 COPY --chown=opam:opam hosts/ocaml/dune-project ./
 COPY --chown=opam:opam hosts/ocaml/lib/ ./lib/
 COPY --chown=opam:opam hosts/ocaml/bin/dune hosts/ocaml/bin/run_tests.ml \
      hosts/ocaml/bin/debug_set.ml hosts/ocaml/bin/sx_server.ml ./bin/
-RUN eval $(opam env) && dune build bin/sx_server.exe
+RUN dune build bin/sx_server.exe
 
 # --- Stage 2: Python app ---
 FROM python:3.11-slim AS base