fix(docker): optimize dockerfile with GOPROXY

Dockerfile

@@ -6,16 +6,17 @@ RUN git config --global url."https://${GITHUB_TOKEN}:x-oauth-basic@github.com/".
 
 # Mark private repositories
 ENV GOPRIVATE=github.com/renproject/darknode
+ENV GOPROXY=https://proxy.golang.org
 
 # Download dependencies.
 WORKDIR /lightnode
 COPY go.mod .
 COPY go.sum .
+RUN go mod edit -replace=github.com/filecoin-project/filecoin-ffi=$(go env GOPATH)/src/github.com/filecoin-project/filecoin-ffi
 RUN go mod download
 
 # Copy the code into the container.
 COPY . .
-RUN go mod edit -replace=github.com/filecoin-project/filecoin-ffi=$(go env GOPATH)/src/github.com/filecoin-project/filecoin-ffi
 
 # Build the code inside the container.
 RUN go build ./cmd/lightnode