add Dockerfile

Dockerfile

@@ -0,0 +1,23 @@
+FROM golang AS builder
+
+WORKDIR /go/src/PR-DNSd
+
+COPY go.mod go.sum ./
+
+RUN go mod download && go mod verify
+
+COPY *.go ./
+
+RUN CGO_ENABLED=0 GOOS=linux \
+  go build -a -installsuffix cgo -o /go/bin/ \
+  -ldflags "-X main.DefaultChroot=" \
+  ./...
+
+FROM scratch
+
+COPY --from=builder /go/bin/* /bin/
+COPY --from=builder /etc/ssl/certs/ca-certificates.crt /etc/ssl/ca-bundle.pem
+
+EXPOSE 53/udp
+
+ENTRYPOINT [ "/bin/PR-DNSd" ]