python3-build: Update Yum repos.

Centos7 is EOL and the mirrorlist is now offline.

images/python3-build/Dockerfile

@@ -3,7 +3,8 @@ FROM registry.hub.docker.com/library/centos:7
 ENV LC_ALL en_US.UTF-8
 ENV LANG en_US.UTF-8
 # hadolint ignore=DL3013,DL3033
-RUN yum group install -y "Development tools" && \
+RUN sed -i '/^mirrorlist=http:\/\/mirrorlist.centos.org\//d; s@^#baseurl=http://mirror.centos.org/@baseurl=http://vault.centos.org/@' /etc/yum.repos.d/*.repo && \
+    yum group install -y "Development tools" && \
     yum install -y \
         python3-devel \
         python3-pip \