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>

run-tests.sh

@@ -11,8 +11,8 @@ set -euo pipefail
 
 cd "$(dirname "$0")"
 
-QUICK=false
-SX_ONLY=false
+QUICK="${QUICK:-false}"
+SX_ONLY="${SX_ONLY:-false}"
 for arg in "$@"; do
   case "$arg" in
     --quick)   QUICK=true ;;