Pin Docker 'builder' stage to current architecture

This is useful for people who want to do multi-architecture Docker
builds; it means the builder stage need only be run omce for all
architectures, even though the final stage needs to be built for *each*
architecture.

Dockerfile

@@ -1,5 +1,5 @@
 # Builder
-FROM node:current as builder
+FROM --platform=$BUILDPLATFORM node:current as builder
 
 WORKDIR /src