Dockerfile - With Defaultport + EXPOSE.Dockerfile

FROM golang:1.5
LABEL maintainer="PercyBolmer@medium"

#ARG is used during the Image build
ARG port=8080
# Assign Environment variabel PORT the value of port. The user running the container can then override this with the -e flag
ENV PORT=${port}

RUN mkdir /app
COPY main.go /app
WORKDIR /app
RUN go build -o helloer .

EXPOSE ${PORT}

CMD [ "/app/helloer" ]