fix arm64 images being incorrecly annotated as amd64

.github/workflows/build-container-ghcr.yml

@@ -22,10 +22,10 @@ jobs:
       # Build per-arch images
       - name: Build amd64 image
         run: |
-          docker build -t ${IMAGE_NAME}:${VERSION}-amd64 -f Dockerfile --build-arg ARCH=amd64 .
+          docker build --platform linux/amd64 -t ${IMAGE_NAME}:${VERSION}-amd64 -f Dockerfile --build-arg ARCH=amd64 .
       - name: Build arm64 image
         run: |
-          docker build -t ${IMAGE_NAME}:${VERSION}-arm64 -f Dockerfile --build-arg ARCH=arm64 .
+          docker build --platform linux/arm64 -t ${IMAGE_NAME}:${VERSION}-arm64 -f Dockerfile --build-arg ARCH=arm64 .
       - name: Push per-arch images
         run: |
           docker push ${IMAGE_NAME}:${VERSION}-amd64

Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.24-alpine AS build
+FROM --platform=$BUILDPLATFORM golang:1.24-alpine AS build
 ARG ARCH=amd64
 WORKDIR /build
 COPY main.go .