split Docker build into phases

- as per https://github.com/gohugoio/hugo/issues/12991

Dockerfile

@@ -3,10 +3,11 @@ ARG HUGO_VERSION="v0.147.1"
 ARG CADDY_VERSION="2.10.0"
 
 # Stage 1: Build
-FROM ghcr.io/gohugoio/hugo:${HUGO_VERSION} AS builder
-WORKDIR /project
+FROM ghcr.io/gohugoio/hugo:${HUGO_VERSION} AS source
 COPY . .
-RUN hugo build
+
+FROM source AS builder
+RUN /entrypoint.sh build
 
 # Stage 2: Package
 FROM docker.io/caddy:${CADDY_VERSION}