diff --git a/run-tests.sh b/run-tests.sh
index 3ee87a2..854d984 100755
--- a/run-tests.sh
+++ b/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 ;;
diff --git a/sx/Dockerfile b/sx/Dockerfile
index c87f68b..ecb4d6d 100644
--- a/sx/Dockerfile
+++ b/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