fix: use TARGETARCH for multi-platform Docker builds

Use Docker's TARGETARCH variable to download the correct architecture
binaries for Go and Hugo during multi-platform builds. This fixes the
build failure for linux/arm64 where amd64 binaries were being downloaded.

- Add ARG TARGETARCH to capture target architecture
- Replace hardcoded linux-amd64 with linux-${TARGETARCH}
- Applies to both Go and Hugo downloads
- Backwards compatible with local single-platform builds

Dockerfile

@@ -6,25 +6,28 @@ ARG HUGO_VERSION=0.151.0
 # Build stage - use same versions as local devbox environment
 FROM node:${NODE_VERSION}-bookworm AS builder
 
+# Get target architecture for multi-platform builds
+ARG TARGETARCH
+
 # Install Git (needed for Hugo's enableGitInfo)
 RUN apt-get update && apt-get install -y git && rm -rf /var/lib/apt/lists/*
 
-# Install Go
+# Install Go (map TARGETARCH: amd64->amd64, arm64->arm64)
 ARG GO_VERSION
-RUN wget -q https://go.dev/dl/go${GO_VERSION}.linux-amd64.tar.gz && \
-    tar -C /usr/local -xzf go${GO_VERSION}.linux-amd64.tar.gz && \
-    rm go${GO_VERSION}.linux-amd64.tar.gz
+RUN wget -q https://go.dev/dl/go${GO_VERSION}.linux-${TARGETARCH}.tar.gz && \
+    tar -C /usr/local -xzf go${GO_VERSION}.linux-${TARGETARCH}.tar.gz && \
+    rm go${GO_VERSION}.linux-${TARGETARCH}.tar.gz
 
 ENV PATH="/usr/local/go/bin:${PATH}"
 ENV GOPATH="/go"
 ENV PATH="${GOPATH}/bin:${PATH}"
 
-# Install Hugo extended
+# Install Hugo extended (map TARGETARCH: amd64->amd64, arm64->arm64)
 ARG HUGO_VERSION
-RUN wget -q https://github.com/gohugoio/hugo/releases/download/v${HUGO_VERSION}/hugo_extended_${HUGO_VERSION}_linux-amd64.tar.gz && \
-    tar -xzf hugo_extended_${HUGO_VERSION}_linux-amd64.tar.gz && \
+RUN wget -q https://github.com/gohugoio/hugo/releases/download/v${HUGO_VERSION}/hugo_extended_${HUGO_VERSION}_linux-${TARGETARCH}.tar.gz && \
+    tar -xzf hugo_extended_${HUGO_VERSION}_linux-${TARGETARCH}.tar.gz && \
     mv hugo /usr/local/bin/ && \
-    rm hugo_extended_${HUGO_VERSION}_linux-amd64.tar.gz && \
+    rm hugo_extended_${HUGO_VERSION}_linux-${TARGETARCH}.tar.gz && \
     hugo version
 
 WORKDIR /src