[docker] Add Dockerfile

Dockerfile

+FROM golang:1.15.6-alpine
+COPY . /src
+WORKDIR /src
+RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build ./cmd/srpmproc
+
+FROM centos:8.3.2011
+COPY --from=0 /src/srpmproc /usr/bin/srpmproc
+
+ENTRYPOINT ["/usr/bin/srpmproc"]