Your URL is wrong, the following worked for me:

RUN curl http://mirror.centos.org/centos/7/os/x86_64/RPM-GPG-KEY-CentOS-7 -o RPM-GPG-KEY-CentOS-7
RUN rpm --import RPM-GPG-KEY-CentOS-7
RUN yum-config-manager --add-repo='http://mirror.centos.org/centos/7/os/x86_64/'

Note that a lot of their packages will conflict with Amazon's pre-installed packages.

One workaround I've found is to not add that repo, and instead use yum install <direct_rpm_link> for all of my installations. I had to manually resolve some dependencies myself by adding more .rpm links, but at least it worked in the end.

Instead of guessing which version of RHEL a particular distro is based off, just run:

rpm -E %{rhel}

For Amazon Linux 2, this will give you 7.

Although the @amzn-main repo doesn't have PHP 7.2 yet (as far as I know), you can use remi-php72. According to his release blog you can install the EPEL and Remi repositories via:

wget https://dl.fedoraproject.org/pub/epel/epel-release-latest-6.noarch.rpm
wget http://rpms.remirepo.net/enterprise/remi-release-6.rpm
rpm -Uvh remi-release-6.rpm
rpm -Uvh epel-release-latest-6.noarch.rpm

And then enable remi-php72 using yum-config-manager:

yum install yum-utils
yum-config-manager --enable remi-php72

After that, you can simply search and install php and all the needed extensions like normal:

sudo yum install --disablerepo="*" --enablerepo="remi,remi-php72" php php-fpm php-cli ... 

I've tested installing a few packages from EPEL for Enterprise Linux 7. Make sure you do use the EL7 version, since Amazon Linux 2 is systemd based:

https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm

amazon-linux-extras only has a few packages for now, so if that's all you need, great. Depending on the packages you install, you always run the risk of conflicts between EPEL and Amazon repos.

you can get the centos/rhel version details from the file "/proc/version"

to get the version detail alone then you can grep like this

echo "Red Hat:`cat /proc/version | awk -F "Red Hat" '{print $2}' | awk -F ")" '{print $1}'`"

else using rpm

rpm -E %{rhel}