Update Dockerfile and CI for multi-platform builds (#6)

* Use docker buildx in CI

* Update Dockerfile for buildx

.github/workflows/ci.yaml

@@ -41,6 +41,9 @@ jobs:
         with:
           images: armsnyder/a2s-exporter
 
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v2
+
       - name: Log in to Docker Hub
         if: github.event_name == 'push' && startsWith(github.ref, 'refs/tags/v')
         uses: docker/login-action@v2

Dockerfile

@@ -1,11 +1,10 @@
+# syntax=docker/dockerfile:1.4
 FROM golang:1.19 AS builder
 WORKDIR /build
-COPY go.mod go.sum ./
-RUN go mod download
-COPY internal internal
-COPY *.go .
+COPY . .
 ARG BUILD_VERSION=development
-RUN GOOS=linux GOARCH=amd64 CGO_ENABLED=0 go build -ldflags="-w -s -X 'main.buildVersion=$BUILD_VERSION'" -o /bin/a2s-exporter .
+ENV CGO_ENABLED=0
+RUN --mount=type=cache,target=/root/.cache/go-build go build -ldflags="-w -s -X 'main.buildVersion=$BUILD_VERSION'" -o /bin/a2s-exporter .
 
 FROM scratch
 COPY --from=builder /bin/a2s-exporter /bin/a2s-exporter