fix: cross-compile natively via BUILDPLATFORM/TARGETARCH instead of QEMU-emulating go build

Dockerfile

@@ -1,9 +1,11 @@
-FROM golang:1.26 AS build
+FROM --platform=$BUILDPLATFORM golang:1.26 AS build
+ARG TARGETOS
+ARG TARGETARCH
 WORKDIR /src
 COPY go.mod go.sum ./
 RUN go mod download
 COPY . .
-RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -trimpath -o /out/kmsvc-server ./cmd/server
+RUN CGO_ENABLED=0 GOOS=$TARGETOS GOARCH=$TARGETARCH go build -trimpath -o /out/kmsvc-server ./cmd/server
 
 FROM gcr.io/distroless/static-debian12:nonroot
 COPY --from=build /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/ca-certificates.crt