Replace usage of mirrorlist with baseurl (#307)

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

@@ -1,7 +1,9 @@
-FROM centos:7
+FROM centos:7.6.1810
 
 ENV SOURCE_DIR /root/source
 
+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 dependencies
 RUN yum install -y \
  bzip2 \