refactoring dockerfile (#563)

Dockerfile

@@ -1,19 +1,6 @@
-FROM golang:alpine as builder
+FROM golang:1.16.0-alpine as build-env
+RUN GO111MODULE=on go get -v github.com/projectdiscovery/nuclei/v2/cmd/nuclei
 
-RUN mkdir -p /app
-WORKDIR /app
-COPY ./v2/go.mod .
-RUN go mod download
-
-COPY . .
-RUN cd ./v2/cmd/nuclei && go build -o nuclei .
-
-FROM alpine
-
-RUN mkdir /app
-RUN adduser --home /app --shell /bin/sh --disabled-password appuser
-COPY --from=builder /app/v2/cmd/nuclei/nuclei /app
-USER appuser
-
-WORKDIR /app
-ENTRYPOINT ["/app/nuclei"]
+FROM alpine:latest
+COPY --from=build-env /go/bin/nuclei /usr/local/bin/nuclei
+ENTRYPOINT ["nuclei"]