Shaved 600mB off resulting Docker image

By removing the build files after the build is complete, we save 600mB of unnecessary bulk.  The total image size will now be 1gB.

Dockerfile

@@ -22,11 +22,10 @@ RUN apt-get update && apt-get install -y \
     wget \
     ca-certificates \
     libpixman-1-dev \
-    && rm -fr /var/lib/apt/lists/*
-RUN mkdir /app
-WORKDIR /app
-COPY . .
+    && rm -rf /var/lib/apt/lists/*
 ARG CC=gcc-9
 ARG CXX=g++-9
 ARG LLVM_CONFIG=llvm-config-9
-RUN make clean && make distrib && make install
+COPY . /app
+RUN cd /app && make clean && make distrib && \
+    make install && cd .. && rm -rf /app