fix(container): cross-compile per target platform so linux/arm64 ships an arm64 binary (#382)

The build stage ran natively on $BUILDPLATFORM and make never received a
target arch, so both manifest variants shipped the same amd64 binary
(#379). Pass TARGETOS/TARGETARCH into the build with CGO_ENABLED=0 and
verify each platform variant runs 'pmg version' after push.

Fixes #379


Claude-Session: https://claude.ai/code/session_01E8yrxjqVw7454zByb8kxvD

Co-authored-by: Claude <noreply@anthropic.com>

.github/workflows/container.yml

@@ -67,4 +67,10 @@ jobs:
             .
         fi
 
+    - name: Verify Image Platform Variants
+      run: |
+        for platform in linux/amd64 linux/arm64; do
+          docker run --rm --platform $platform $REGISTRY/$IMAGE_NAME:latest version
+        done
+
 

Dockerfile

@@ -9,7 +9,9 @@ RUN go mod download
 
 COPY . .
 
-RUN make
+ARG TARGETOS TARGETARCH
+
+RUN CGO_ENABLED=0 GOOS=${TARGETOS} GOARCH=${TARGETARCH} make
 
 FROM debian:11-slim@sha256:e4b93db6aad977a95aa103917f3de8a2b16ead91cf255c3ccdb300c5d20f3015
 # Original: debian:11-slim