Convert to multi-stage Dockerfile.

- Remove architecture-specific Dockerfile and Makefile target.
- Convert Dockerfile to a multi-stage build, keeping only what is needed to run.

Dockerfile

@@ -4,29 +4,42 @@
 # change is that the Hugo version is now an overridable argument rather than a fixed
 # environment variable.
 
-FROM golang:1.15-alpine
+FROM golang:1.16-alpine
 
 LABEL maintainer="Luc Perkins <lperkins@linuxfoundation.org>"
 
 RUN apk add --no-cache \
     curl \
-    git \
-    openssh-client \
-    rsync \
+    gcc \
+    g++ \
+    musl-dev \
     build-base \
-    libc6-compat \
-    npm && \
-    npm install -D autoprefixer postcss-cli
+    libc6-compat
 
 ARG HUGO_VERSION
 
+RUN mkdir $HOME/src && \
+    cd $HOME/src && \
+    curl -L https://github.com/gohugoio/hugo/archive/refs/tags/v${HUGO_VERSION}.tar.gz | tar -xz && \
+    cd "hugo-${HUGO_VERSION}" && \
+    go install --tags extended
+
+FROM golang:1.16-alpine
+
+RUN apk add --no-cache \
+    git \
+    openssh-client \
+    rsync \
+    npm && \
+    npm install -D autoprefixer postcss-cli
+
 RUN mkdir -p /usr/local/src && \
     cd /usr/local/src && \
-    curl -L https://github.com/gohugoio/hugo/releases/download/v${HUGO_VERSION}/hugo_extended_${HUGO_VERSION}_Linux-64bit.tar.gz | tar -xz && \
-    mv hugo /usr/local/bin/hugo && \
     addgroup -Sg 1000 hugo && \
     adduser -Sg hugo -u 1000 -h /src hugo
 
+COPY --from=0 /go/bin/hugo /usr/local/bin/hugo
+
 WORKDIR /src
 
 USER hugo:hugo