I just learned docker and asked two questions about writing dockefile.

1. Many examples written by dockerfile on the Internet are based on ubuntu,apt-get installation, and centos yum installation is rare. Why?
2, is also an example, such as

# Add the package verification key
 RUN apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 7F0CEB10

I know this sentence is to add keys, and some keys are longer (573bfd6b3d8fbc641079a6ababf5bd827bd9bf62). I want to know where these keys are generated.

Thank you