chore(release) use Python 3.8+ in older distributions

Fixing V8 build issues, see NVIDIA/DeepLearningExamples#1016

Fixes #430

Author: thibaultcha

assets/release/Dockerfiles/Dockerfile.amd64.centos7

@@ -1,8 +1,11 @@
-FROM amd64/centos:7
+# Use Python 3.8+ (required by V8's GN build system)
+# See https://hub.docker.com/r/centos/python-38-centos7
+FROM centos/python-38-centos7
 COPY . /ngx_wasm_module
+USER root
 
-RUN yum install -y epel-release && \
-    yum update -y && \
+RUN yum update -y && \
+    yum install -y epel-release && \
     yum install -y centos-release-scl && \
     yum install -y \
         devtoolset-8 \
@@ -13,16 +16,16 @@ RUN yum install -y epel-release && \
         cmake3 \
         make \
         gcc \
-        python3 \
         git \
         glib2-devel \
         perl \
         perl-IPC-Cmd \
         perl-Test-Simple
 
+RUN ln -nfs $(which cmake3) /usr/bin/cmake
+
 ENV RUSTUP_HOME=/usr/local/rustup CARGO_HOME=/usr/local/cargo
 ENV PATH $CARGO_HOME/bin:$PATH
-RUN ln -nfs $(which cmake3) /usr/bin/cmake
 RUN mkdir -p "$CARGO_HOME" && mkdir -p "$RUSTUP_HOME" && \
     curl https://sh.rustup.rs -sSf | sh -s -- -y --default-toolchain stable && \
     chmod -R a=rwX $CARGO_HOME $RUSTUP_HOME

assets/release/Dockerfiles/Dockerfile.amd64.centos8

@@ -13,7 +13,7 @@ RUN sed -i 's/mirrorlist/#mirrorlist/g' /etc/yum.repos.d/CentOS-Linux-* && \
         libstdc++-devel \
         libatomic \
         perl \
-        python3 \
+        python39 \
         git \
         glib2-devel
 

assets/release/Dockerfiles/Dockerfile.amd64.ubuntu-18.04

@@ -2,13 +2,20 @@ FROM amd64/ubuntu:18.04
 COPY . /ngx_wasm_module
 
 ARG DEBIAN_FRONTEND=noninteractive
+
+# Install Python 3.8+ (required by V8's GN build system)
+RUN apt-get update && \
+    apt-get install -y software-properties-common && \
+    add-apt-repository -y ppa:deadsnakes/ppa && \
+    apt-get update && \
+    apt-get install -y python3.8 && \
+    ln -nfs $(which python3.8) /usr/bin/python3
+
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \
         ca-certificates \
         build-essential \
         ninja-build \
-        python3 \
-        cmake \
         gcc-8 \
         libstdc++-8-dev \
         git \