Update docker file

Author: Ivaylo Korakov

Dockerfile

@@ -1,14 +1,26 @@
-FROM golang:1.8
+# Builder stage to
+FROM golang:1.12 as builder
+
+# Add the project in the image
 ADD . /go/src/github.com/kumina/postfix_exporter
 WORKDIR /go/src/github.com/kumina/postfix_exporter
-RUN apt-get update -qq && apt-get install -qqy \
+
+# Install needed dependencies for the build
+RUN apt-get update -q && apt-get install -qy \
   build-essential \
   libsystemd-dev
-RUN go get -v ./...
-RUN go build
 
+# Get dependencies and build the static binary
+RUN go get -d ./...
+RUN go build -a -tags static_all
+
+# Real image
 FROM debian:latest
+
 EXPOSE 9154
 WORKDIR /
-COPY --from=0 /go/src/github.com/kumina/postfix_exporter/postfix_exporter .
+
+# Copy the binary from the build image to the real one
+COPY --from=builder /go/src/github.com/kumina/postfix_exporter/postfix_exporter .
+
 ENTRYPOINT ["/postfix_exporter"]