Refactor Dockerfile according to best practices (#4)

* Refactor Dockerfile according to best practices
* Bump bind9 and dnsutils version to deb9u6
#patch

Author: wernerfred

Dockerfile

@@ -1,19 +1,20 @@
-FROM golang:latest as builder 
+FROM golang:1.14.1 as builder 
 RUN mkdir /build 
-ADD api/* /build/ 
+COPY api/* /build/ 
 WORKDIR /build
 RUN go get -d -v ./
 RUN go build -o dyndns-api . 
 
 FROM debian:stretch
 ARG DEBIAN_FRONTEND=noninteractive
-RUN apt update -qq && \
-    apt install -q -y bind9 dnsutils -qq && \
-    apt-get clean
+RUN apt-get update -qq && \
+    apt-get install -q -y bind9=1:9.10.3.dfsg.P4-12.3+deb9u6 dnsutils=1:9.10.3.dfsg.P4-12.3+deb9u6 -qq --no-install-recommends && \
+    apt-get clean && \
+    rm -rf /var/lib/apt/lists/*
 
 COPY --from=builder /build/dyndns-api /root/
 COPY ./setup.sh /root/setup.sh
 RUN chmod +x /root/setup.sh 
 
 EXPOSE 53 8080
-ENTRYPOINT ["/root/setup.sh"]
+ENTRYPOINT ["/root/setup.sh"]