Use Docker's multi-stage builds

Author: tetsuharuohzeki

Dockerfile

@@ -2,4 +2,9 @@ FROM golang:1.14-buster
 ADD . /gh-action-detect-unmergeable/
 WORKDIR /gh-action-detect-unmergeable/
 RUN ["make", "build", "-j"]
-ENTRYPOINT ["/gh-action-detect-unmergeable/ghaction_unmergeable_detection"]
+
+FROM debian:buster-20200607-slim
+WORKDIR /root/
+COPY --from=builder /gh-action-detect-unmergeable/ghaction_unmergeable_detection .
+RUN apt-get update && apt-get install -y --no-install-recommends ca-certificates && rm -rf /var/lib/apt/lists/*
+ENTRYPOINT ["/root/ghaction_unmergeable_detection"]