Replace usage of mirrorlist with baseurl (#250)

Motivation:

We can't use the default mirrorlist anymore as it will fail and so fail
the docker build

Modifications:

Use baseurl with the correct vault

Result:

Docker image can be build again

docker/Dockerfile.centos7

@@ -1,10 +1,13 @@
-FROM centos:7.9.2009
+FROM centos:7.6.1810
 
 ARG gcc_version=10.2-2020.11
 ENV GCC_VERSION $gcc_version
 ENV MAVEN_VERSION 3.9.1
 ENV SOURCE_DIR /root/source
 
+# Update as we need to use the vault now.
+RUN sed -i -e 's/^mirrorlist/#mirrorlist/g' -e 's/^#baseurl=http:\/\/mirror.centos.org\/centos\/$releasever\//baseurl=https:\/\/vault.centos.org\/\/7.6.1810\//g' /etc/yum.repos.d/CentOS-Base.repo
+
 # Install requirements
 RUN yum install -y \
 	apr-devel \

docker/Dockerfile.centos7arm64v8

@@ -1,7 +1,10 @@
-FROM arm64v8/centos:7.9.2009
+FROM arm64v8/centos:7.6.1810
 
 ENV MAVEN_VERSION 3.9.1
 
+# Update as we need to use the vault now.
+RUN sed -i -e 's/^mirrorlist/#mirrorlist/g' -e 's/^#baseurl=http:\/\/mirror.centos.org\/centos\/$releasever\//baseurl=https:\/\/vault.centos.org\/\/7.6.1810\//g' /etc/yum.repos.d/CentOS-Base.repo
+
 # Install requirements
 RUN yum install -y \
 	apr-devel \