build: separate to-json compile

Dockerfile

@@ -38,16 +38,21 @@ RUN env CGO_ENABLED=0 go build -v -trimpath -ldflags "-X main.VersionString=${DN
 COPY --from=upx /tmp/upx /usr/bin/
 RUN upx -9 /tmp/dnsproxy
 
-FROM ${RUST} AS cleardns
-RUN apk add build-base cmake
-COPY ./ /ClearDNS/
-WORKDIR /ClearDNS/src/to-json/
+FROM ${RUST} AS to-json
+# TODO: update crates.io index
+COPY ./src/to-json/ /to-json/
+WORKDIR /to-json/
 RUN cargo build --release
-WORKDIR /ClearDNS/bin/
+WORKDIR ./target/release/
+RUN cp libto_json.a /
+
+FROM ${ALPINE} AS cleardns
+RUN apk add build-base cmake
+COPY ./ /cleardns/
+COPY --from=to-json /libto_json.a /cleardns/src/to-json/target/release/
+WORKDIR /cleardns/bin/
 RUN cmake -DCMAKE_EXE_LINKER_FLAGS=-static -DCMAKE_BUILD_TYPE=Release .. && make
 RUN strip cleardns && mv cleardns /tmp/
-COPY --from=upx /tmp/upx /usr/bin/
-RUN upx -9 /tmp/cleardns
 
 FROM ${ALPINE} AS build
 RUN apk add xz