Let the capture script take a host-resolver mapping

frontend/scripts/capture-screenshots.mjs

@@ -37,7 +37,13 @@ async function withTheme(browser, theme) {
   return context
 }
 
-const browser = await chromium.launch()
+// Chromium pins *.localhost to loopback (RFC 6761), so /etc/hosts cannot point
+// it at Traefik. The containerised run (root `make verify-docker`) passes the
+// mapping through here instead; empty on a host run.
+const RESOLVER = process.env.HOST_RESOLVER_RULES
+const browser = await chromium.launch(
+  RESOLVER ? { args: [`--host-resolver-rules=${RESOLVER}`] } : {},
+)
 
 for (const theme of ["light", "dark"]) {
   const dir = `${OUT}/${theme}`