Allow to pass custom LHCI config

The path of the custom config file is not yet configurable and needs to be
'.github/lighthouserc-custom.yml'.

entrypoint.sh

@@ -167,6 +167,7 @@ query_string="?_fd=0&pb=0"
 min_score_performance="${LHCI_MIN_SCORE_PERFORMANCE:-0.6}"
 min_score_accessibility="${LHCI_MIN_SCORE_ACCESSIBILITY:-0.9}"
 
+# Prepare the default CI Lighthouse config
 cat <<- EOF > lighthouserc.yml
 ci:
   collect:
@@ -195,6 +196,11 @@ ci:
           aggregationMethod: median-run
 EOF
 
+# Merge custom Lighthouse CI config, if provided
+if [[ -f ".github/lighthouserc-custom.yml" ]]; then
+  yq eval-all 'select(fileIndex == 0) * select(fileIndex == 1)' lighthouserc.yml .github/lighthouserc-custom.yml -i
+fi
+
 cat <<-EOF > setPreviewCookies.js
 module.exports = async (browser) => {
   // launch browser for LHCI