Added a multi-stage Dockerfile for easier local image builds

Dockerfile

@@ -1,8 +1,13 @@
+FROM golang:1.17 as builder
+
+WORKDIR /build
+
+COPY go.mod go.sum ./
+RUN go mod download
+
+COPY . .
+RUN CGO_ENABLED=0 go build ./cmd/mc-router
+
 FROM scratch
-
-LABEL org.opencontainers.image.authors="Geoff Bourne <itzgeoff@gmail.com>"
-LABEL org.opencontainers.image.title="mc-router"
-LABEL org.opencontainers.image.source="https://github.com/itzg/mc-router"
-
-COPY mc-router /
 ENTRYPOINT ["/mc-router"]
+COPY --from=builder /build/mc-router /mc-router