fix(release): remove TARGETPLATFORM indirection from goreleaser Dockerfile

goreleaser builds each arch image in a separate pass, placing only that
arch's binary at the root of the build context as plain "mantle".
The COPY $TARGETPLATFORM/mantle pattern is for manual multi-arch builds
where all binaries coexist in one context — it doesn't apply here.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: 2 people

packages/engine/Dockerfile.goreleaser

@@ -4,8 +4,7 @@ RUN apk add --no-cache ca-certificates tzdata \
     && addgroup -S mantle \
     && adduser -S -u 10001 -G mantle -h /home/mantle -s /bin/sh mantle
 
-ARG TARGETPLATFORM
-COPY $TARGETPLATFORM/mantle /usr/local/bin/mantle
+COPY mantle /usr/local/bin/mantle
 
 ENV MANTLE_LOG_LEVEL=info
 ENV MANTLE_API_ADDRESS=:8080