ci: Build multi-platform image with cross-compilation

The previous build workflow used emulation to build the Docker image,
which results in a somewhat complicated push-by-digest and merge
workflow to create a multi-platform image.

This commit changes the Docker build to use cross-compilation instead,
resulting in a faster and more straightforward build.

Signed-off-by: David Karlsson <35727626+dvdksn@users.noreply.github.com>

Dockerfile

@@ -2,11 +2,17 @@
 # Twitter:      https://twitter.com/gohugoio
 # Website:      https://gohugo.io/
 
-FROM golang:1.22.6-alpine AS build
+FROM --platform=$BUILDPLATFORM tonistiigi/xx:1.5.0 AS xx
+
+FROM --platform=$BUILDPLATFORM golang:1.22.6-alpine AS build
+
+# Set up cross-compilation helpers
+COPY --from=xx / /
+RUN apk add clang lld
 
 # Optionally set HUGO_BUILD_TAGS to "extended" or "nodeploy" when building like so:
 #   docker build --build-arg HUGO_BUILD_TAGS=extended .
-ARG HUGO_BUILD_TAGS
+ARG HUGO_BUILD_TAGS="none"
 
 ARG CGO=1
 ENV CGO_ENABLED=${CGO}
@@ -15,20 +21,26 @@ ENV GO111MODULE=on
 
 WORKDIR /go/src/github.com/gohugoio/hugo
 
-COPY . /go/src/github.com/gohugoio/hugo/
+RUN --mount=src=go.mod,target=go.mod \
+    --mount=src=go.sum,target=go.sum \
+    --mount=type=cache,target=/go/pkg/mod \
+    go mod download
 
+ARG TARGETPLATFORM
 # gcc/g++ are required to build SASS libraries for extended version
-RUN apk update && \
-    apk add --no-cache gcc g++ musl-dev git && \
-    go install github.com/magefile/mage
-
-RUN mage hugo && mage install
+RUN xx-apk add --no-scripts --no-cache gcc g++ musl-dev git
+RUN --mount=target=. \
+    --mount=type=cache,target=/go/pkg/mod <<EOT
+    set -ex
+    xx-go build -tags "$HUGO_BUILD_TAGS" -o /usr/bin/hugo
+    xx-verify /usr/bin/hugo
+EOT
 
 # ---
 
 FROM alpine:3.18
 
-COPY --from=build /go/bin/hugo /usr/bin/hugo
+COPY --from=build /usr/bin/hugo /usr/bin/hugo
 
 # libc6-compat & libstdc++ are required for extended SASS libraries
 # ca-certificates are required to fetch outside resources (like Twitter oEmbeds)