upgrade rel image builder from ubuntu 16 to 20 (#52)

* upgrade rel image builder from ubuntu 16 to 20

* Update vw-rel-alpine.Dockerfile

Author: jackgerrits

vowpal_wabbit/vw-rel-alpine.Dockerfile

@@ -1,10 +1,10 @@
-FROM vowpalwabbit/ubuntu1604-build AS build
-ARG branch_or_tag=8.8.0
+FROM vowpalwabbit/ubuntu2004-build AS build
+ARG branch_or_tag=9.4.0
 RUN git clone -b $branch_or_tag --depth=1 --recursive https://github.com/VowpalWabbit/vowpal_wabbit.git /vw
-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 $(nproc)
+
+WORKDIR /vw
+RUN cmake -S . -B build -G Ninja -DCMAKE_BUILD_TYPE=Release -DWARNINGS=Off -DSTATIC_LINK_VW=On -DBUILD_JAVA=Off -DBUILD_PYTHON=Off -DBUILD_TESTING=Off
+RUN cmake --build build -t vw_cli_bin
 
 FROM alpine:latest
 COPY --from=build /vw/build/vowpalwabbit/cli/vw .