Use Go's native cross-compile support instead of emulation

Signed-off-by: Nic Cope <[email protected]>

Dockerfile

@@ -2,23 +2,24 @@
 
 # We use the latest Go 1.x version unless asked to use something else.
 ARG GO_VERSION=1
+ARG TARGETOS
+ARG TARGETARCH
 
 # Setup the base environment.
-FROM golang:${GO_VERSION} AS base
+FROM --platform=${BUILDPLATFORM} golang:${GO_VERSION} AS base
 
 WORKDIR /fn
 ENV CGO_ENABLED=0
 
 COPY go.mod go.sum ./
-RUN --mount=type=cache,target=/go/pkg/mod \
-    go mod download
+RUN --mount=type=cache,target=/go/pkg/mod go mod download
 
 # Build the Function.
 FROM base AS build
 RUN --mount=target=. \
     --mount=type=cache,target=/go/pkg/mod \
     --mount=type=cache,target=/root/.cache/go-build \
-    go build -o /function .
+    GOOS=${TARGETOS} GOARCH=${TARGETARCH} go build -o /function .
 
 # Produce the Function image.
 FROM gcr.io/distroless/base-debian11 AS image