Use multi-line in Amazon and CentOS builders

The following discussion from !206 (merged) should be addressed:

• @rmarshall started a discussion:

  This should probably be multi-line like the others, yes?

  RUN amazon-linux-extras install epel -y \
      && yum install -y yum-utils \
      && yum-config-manager --enable epel

We will address this for Amazon and CentOS 8 both as a follow up.