diff --git a/Dockerfile b/Dockerfile
index d46a764..7c7879b 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -19,6 +19,13 @@ COPY . .
 
 RUN chmod +x /app/start.sh
 
+# Locate sitespeed.js at build time and write the path to /sitespeed_env.
+# The base image sets sitespeed.io as ENTRYPOINT via a full path, so it is
+# not on $PATH. We find it once here so start.sh can export it at runtime.
+RUN SITESPEED_JS=$(find / -name 'sitespeed.js' -path '*/bin/*' 2>/dev/null | head -1); \
+    echo "Build-time sitespeed.js found at: $SITESPEED_JS"; \
+    echo "export SITESPEED_BIN=$SITESPEED_JS" > /sitespeed_env
+
 # Create persistent directories
 RUN mkdir -p /data/reports
 
diff --git a/runner.js b/runner.js
index b1f0ed4..ae4b0cd 100644
--- a/runner.js
+++ b/runner.js
@@ -6,10 +6,6 @@ import { existsSync } from 'fs';
 const __dirname = dirname(fileURLToPath(import.meta.url));
 const LOCAL_BIN = join(__dirname, '..', 'sitespeed.io', 'bin', 'sitespeed.js');
 
-// Shell-escape a single argument (single-quote wrapping)
-function q(arg) {
-  return `'${String(arg).replace(/'/g, "'\\''")}'`;
-}
 
 export function runTest(job, onLine) {
   return new Promise((resolve, reject) => {
@@ -40,12 +36,17 @@ export function runTest(job, onLine) {
     let child;
 
     if (isDocker) {
-      // In Docker, sitespeed.io is on PATH but execSync can't see it from Node's
-      // limited environment. Use 'sh -c' so the full shell PATH is used.
-      const shellCmd = ['sitespeed.io', ...sitespeedArgs].map(q).join(' ');
-      onLine(`[runner] sh -c ${shellCmd.slice(0, 120)}...`);
+      // SITESPEED_BIN is set by start.sh from the build-time path discovery
+      const bin = process.env.SITESPEED_BIN;
+      if (!bin) {
+        return reject(new Error(
+          'SITESPEED_BIN is not set. The Docker build may not have found sitespeed.js.\n' +
+          'Check build logs for "Build-time sitespeed.js found at:"'
+        ));
+      }
+      onLine(`[runner] node ${bin}`);
       onLine(`[runner] DISPLAY=${env.DISPLAY}`);
-      child = spawn('sh', ['-c', shellCmd], { cwd: __dirname, env });
+      child = spawn('node', [bin, ...sitespeedArgs], { cwd: __dirname, env });
     } else {
       if (!existsSync(LOCAL_BIN)) {
         return reject(new Error(
diff --git a/start.sh b/start.sh
index 696dac1..2150a7c 100644
--- a/start.sh
+++ b/start.sh
@@ -1,12 +1,17 @@
 #!/bin/bash
 set -e
 
-# Start virtual framebuffer so Chrome/Firefox have a display to render into.
-# This is what the base sitespeedio/sitespeed.io image's entrypoint does.
+# Load the sitespeed.io binary path discovered at build time
+if [ -f /sitespeed_env ]; then
+  source /sitespeed_env
+fi
+
+echo "[start.sh] SITESPEED_BIN=${SITESPEED_BIN:-NOT FOUND}"
+
+# Start virtual framebuffer so Chrome/Firefox have a display to render into
 Xvfb :99 -ac -screen 0 1920x1080x24 +extension RANDR &
 export DISPLAY=:99
 
-# Give Xvfb a moment to initialise
 sleep 1
 
 exec node /app/app.js