Add plugin folder to Dockerfile

gateway/Dockerfile.build

@@ -10,6 +10,7 @@ COPY requests       requests
 COPY tests          tests
 COPY server.go      .
 COPY types          types
+COPY plugin         plugin
 
 RUN go test -v ./tests && \
     CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o app .