diff --git a/backend/Dockerfile b/backend/Dockerfile
index f9af4c3..257756b 100644
--- a/backend/Dockerfile
+++ b/backend/Dockerfile
@@ -8,7 +8,10 @@
 # default is the minimal engine.
 
 # ── Dashboard ────────────────────────────────────────────────────
-FROM oven/bun:1 AS frontend
+# Built on the build machine's own CPU: the output is the same for every
+# platform, and under emulation for an arm64 image this stage alone takes
+# half an hour.
+FROM --platform=$BUILDPLATFORM oven/bun:1 AS frontend
 
 WORKDIR /app
 COPY package.json bun.lock /app/