Update vw-rel-alpine.Dockerfile

Author: bassmang

vowpal_wabbit/vw-rel-alpine.Dockerfile

@@ -4,7 +4,7 @@ RUN git clone -b $branch_or_tag --depth=1 --recursive https://github.com/VowpalW
 WORKDIR vw
 WORKDIR build
 RUN cmake .. -DCMAKE_BUILD_TYPE=Release -DWARNINGS=Off -DSTATIC_LINK_VW=On -DBUILD_JAVA=Off -DBUILD_PYTHON=Off -DBUILD_TESTING=Off
-RUN make vw_cli_bin -j $(cat nprocs.txt)
+RUN make vw_cli_bin -j $(nproc)
 
 FROM alpine:latest
 COPY --from=build /vw/build/vowpalwabbit/cli/vw .