docker: Use go get to fetch dependencies

- Use proper GOPATH to avoid recurring dependencies

Dockerfile

@@ -1,18 +1,19 @@
 FROM golang:alpine as build-env
 LABEL MAINTAINER Naveen Kumar Sangi <naveenkumarsangi@protonmail.com>
 
-RUN apk --no-cache add curl git
-RUN curl https://raw.githubusercontent.com/golang/dep/master/install.sh | sh
-ENV SOURCE /go/src/app
+RUN apk --no-cache add git
+ENV SOURCE /go/src/gitlab.com/nkprince007/listen
+RUN mkdir -p $SOURCE
 ADD . $SOURCE
 WORKDIR $SOURCE
-RUN dep ensure -v
+RUN go get -v ./...
 RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o app .
 
 
 FROM alpine:latest
 
 RUN apk --no-cache add ca-certificates
+ENV SOURCE /go/src/gitlab.com/nkprince007/listen
 WORKDIR /root/
-COPY --from=build-env /go/src/app .
+COPY --from=build-env $SOURCE .
 CMD ["./app"]