Fix compiler and linker flags.

- `ARG x y` is poorly defined, so let's avoid it.
- Set ldflags for a release build (strip, omit full paths to source
  files from host system, etc.)

Dockerfile

@@ -4,16 +4,20 @@ ARG go_tag_suffix=-alpine
 
 
 FROM --platform=$TARGETPLATFORM ${go_registry}golang:${go_version}${go_tag_suffix} AS builder
-ARG TARGETARCH TARGETOS
-ARG GOARCH=$TARGETARCH GOOS=$TARGETOS
+ARG TARGETARCH
+ARG TARGETOS
+ARG GOARCH=$TARGETARCH
+ARG GOOS=$TARGETOS
 ARG CGO_ENABLED=0
+ARG GOFLAGS="-trimpath"
+ARG go_ldflags="-s -w"
 
 WORKDIR /src
 COPY go.mod go.sum ./
 RUN go mod download
 
 COPY . ./
-RUN go build -o /bin/govuk-exporter main.go
+RUN go build -o /bin/govuk-exporter -ldflags="$go_ldflags" main.go
 
 
 FROM --platform=$TARGETPLATFORM scratch