fix: discover sitespeed.js path at build time, export via /sitespeed_env

sitespeed.io is not on PATH in the container -- the base image uses a
full absolute path for its ENTRYPOINT. We now run 'find' during the
Docker build, write the path to /sitespeed_env, and source it in
start.sh so SITESPEED_BIN is set before node app.js starts.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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