Update Dockerfile

[prince-kz]

14: curl#6 - "Could not resolve host: mirrorlist.centos.org; Unknown error" trouble resolved with change to centos repos

[prince-kz]

Seems to be good:
"=> [otrs 2/9] RUN sed -i s/mirror.centos.org/vault.centos.org/g /etc/yum.repos.d/CentOS* 1.4s
=> [otrs 3/9] RUN sed -i s/^#.baseurl=http/baseurl=http/g /etc/yum.repos.d/CentOS 1.4s
=> [otrs 4/9] RUN sed -i s/^mirrorlist=http/#mirrorlist=http/g /etc/yum.repos.d/CentOS* 1.2s
=> [otrs 5/9] RUN yum install -y yum-plugin-fastestmirror && yum install -y epel-release && yum -y install https://harbottle.gitlab.io/epmel/7/x86_64/epmel-release.rpm && yum update -y && yum -y install 900.5s
=> [otrs 6/9] COPY *.sh / 0.9s
=> "