Clean up Dockerfile

Dockerfile

@@ -6,8 +6,9 @@ RUN : \
  && apt-get autoremove -y \
  && rm -rf /var/lib/apt/lists/*
 RUN git clone --depth 1 -b v1.17 https://github.com/micropython/micropython.git /src
-RUN cd /src/mpy-cross && make
-RUN cd /src/ports/unix && make submodules && make
-RUN cd /src/ports/esp32 && bash -c "source /opt/esp/idf/export.sh && make"
+WORKDIR /src
+RUN make -C mpy-cross
+WORKDIR /src/ports/esp32
+RUN bash -c "source /opt/esp/idf/export.sh && make submodules && make"
 RUN rm /src/ports/esp32/build-GENERIC/frozen_content.c
-CMD /src/ports/unix/micropython
+CMD make && cp build-GENERIC/firmware.bin /build/