Refactor Docker build.

- Consolidate RUN commands, so as to have fewer container layers.
- Explicitly specify the version for VMOD re2.

Dockerfile

 FROM centos:centos7
 
-RUN yum install -y epel-release
-
-RUN yum update -y -q
-
 COPY varnishcache_varnish60.repo /etc/yum.repos.d/
 
-RUN yum -q makecache -y --disablerepo='*' --enablerepo='varnishcache_varnish60'
-
-RUN yum-config-manager --add-repo https://pkg.uplex.de/rpm/7/uplex-varnish/x86_64/
-
-RUN yum install -y -q varnish-6.0.1
-
-RUN yum install -y -q --nogpgcheck vmod-re2
+RUN yum install -y epel-release && yum update -y -q && \
+    yum -q makecache -y --disablerepo='*' --enablerepo='varnishcache_varnish60' && \
+    yum-config-manager --add-repo https://pkg.uplex.de/rpm/7/uplex-varnish/x86_64/ && \
+    yum install -y -q varnish-6.0.1 && \
+    yum install -y -q --nogpgcheck vmod-re2-1.5.1
 
 COPY k8s-ingress varnish/vcl/vcl.tmpl /