Dockerfile: use native nodejs 20 LTS for build as well

Thanks @pabra

Dockerfile

@@ -1,6 +1,8 @@
 # As a workaround we have to build on nodejs 18
 # nodejs 20 hangs on build with armv6/armv7
-FROM docker.io/library/node:18-alpine AS build_node_modules
+ARG BASE_IMAGE=node:20-alpine
+
+FROM --platform=$BUILDPLATFORM ${BASE_IMAGE} AS build_node_modules
 
 # Update npm to latest
 RUN npm install -g npm@latest
@@ -13,7 +15,7 @@ RUN npm ci --omit=dev &&\
 
 # Copy build result to a new image.
 # This saves a lot of disk space.
-FROM docker.io/library/node:20-alpine
+FROM --platform=$BUILDPLATFORM ${BASE_IMAGE}
 HEALTHCHECK CMD /usr/bin/timeout 5s /bin/sh -c "/usr/bin/wg show | /bin/grep -q interface || exit 1" --interval=1m --timeout=5s --retries=3
 COPY --from=build_node_modules /app /app