Change centos7 to use the vault repo

test/Dockerfile-centos7

@@ -1,5 +1,10 @@
 FROM centos:centos7
 
+# Centos 7 is EOL and is no longer available from the usual mirrors, so switch to https://vault.centos.org
+RUN sed -i 's/enabled=1/enabled=0/g' /etc/yum/pluginconf.d/fastestmirror.conf && \
+    sed -i 's/^mirrorlist/#mirrorlist/g' /etc/yum.repos.d/*.repo && \
+    sed -i 's;^#baseurl=http://mirror;baseurl=https://vault;g' /etc/yum.repos.d/*.repo
+
 # Update and install required software
 RUN yum -y update
 RUN yum -y install epel-release