Remove yum update -y in CentOS Dockerfile to avoid VM and container image inconsistency

[ssst0n3]

The line RUN yum update -y in centos.Dockerfile introduces a potential mismatch between the VM image and the container image. Performing a system update directly in the Dockerfile can lead to unexpected behavior and inconsistencies.

Instead of updating packages by default, remove RUN yum update -y from the Dockerfile. This ensures a more predictable environment, making the resulting image closer to the VM configuration. If users require additional updates or specific packages, they should install them explicitly in their own container images.

[ssst0n3]

- RUN yum update -y

RUN yum install -y \
+ {{ if le $version 8 }}
+     grubby centos-linux-release \
+ {{ end }}
    kernel \