Don't build python for no reason. Copy it (#1690)

docker/benchmark-builder/Dockerfile

@@ -14,6 +14,9 @@
 
 ARG parent_image
 
+# Using multi-stage build to copy latest Python 3.
+FROM gcr.io/fuzzbench/base-image AS base-image
+
 FROM $parent_image
 
 ARG fuzzer
@@ -24,36 +27,26 @@ ENV FUZZER $fuzzer
 ENV BENCHMARK $benchmark
 ENV DEBUG_BUILDER $debug_builder
 
-# Python 3.10.8 is not the default version in Ubuntu 20.04 (Focal Fossa).
-ENV PYTHON_VERSION 3.10.8
 
 # Install dependencies required by Python3 or Pip3.
 RUN apt-get update && \
     apt-get upgrade -y && \
     apt-get install -y \
     curl \
     xz-utils \
-    build-essential \
     zlib1g-dev \
     libssl-dev \
     libffi-dev
 
-RUN cd /tmp/ && \
-    curl -O \
-    https://www.python.org/ftp/python/$PYTHON_VERSION/Python-$PYTHON_VERSION.tar.xz && \
-    tar -xvf Python-$PYTHON_VERSION.tar.xz > /dev/null && \
-    cd Python-$PYTHON_VERSION && \
-    ./configure \
-        --enable-loadable-sqlite-extensions \
-        --enable-optimizations \
-        > /dev/null && \
-    make -j install > /dev/null && \
-    rm -r /tmp/Python-$PYTHON_VERSION.tar.xz /tmp/Python-$PYTHON_VERSION
-
-# Install common python dependencies.
-COPY ./requirements.txt /tmp
-RUN pip3 install -r /tmp/requirements.txt
+RUN rm -rf /usr/local/bin/python3.8* /usr/local/bin/pip3 /usr/local/lib/python3.8 \
+    /usr/local/include/python3.8 /usr/local/lib/python3.8/site-packages
 
+# Copy latest python3 from base-image into local.
+COPY --from=base-image /usr/local/bin/python3* /usr/local/bin/
+COPY --from=base-image /usr/local/bin/pip3* /usr/local/bin/
+COPY --from=base-image /usr/local/lib/python3.10 /usr/local/lib/python3.10
+COPY --from=base-image /usr/local/include/python3.10 /usr/local/include/python3.10
+COPY --from=base-image /usr/local/lib/python3.10/site-packages /usr/local/lib/python3.10/site-packages
 
 # Copy the entire fuzzers directory tree to allow for module dependencies.
 COPY fuzzers $SRC/fuzzers