chore(release) fix CentOS 7 build image

A new issue since CentOS 7 has reached EOL a few days ago. We will
likely drop this but this commit is a fix for posterity should we have
to dig the image back.

assets/release/Dockerfiles/Dockerfile.amd64.centos7

@@ -4,7 +4,10 @@ FROM centos/python-38-centos7
 COPY . /ngx_wasm_module
 USER root
 
-RUN yum update -y && \
+RUN sed -i 's/mirrorlist/#mirrorlist/g' /etc/yum.repos.d/*.repo && \
+    sed -i 's|#baseurl=http://mirror.centos.org|baseurl=http://vault.centos.org|g' /etc/yum.repos.d/*.repo && \
+    yum-config-manager --disable centos-sclo-sclo && \
+    yum update -y && \
     yum install -y epel-release && \
     yum install -y centos-release-scl && \
     yum install -y \