Regarding the issue that has not been changed to docker containers

CentOS image on docker Among them, I am installing rails, mysql, etc. (I am not setting rails or mysql in dockerfile).I just use it like vmware.)

Then

ruby-v
ruby 2.3.0p0 (2015-12-25 revision 53290) [x86_64-linux]

What was supposed to be is

Docker out (ctrl+pctrl+q) and
Launch docker again (docker restart)
Previous

ruby-v
bash:ruby:command not found

is the case.Why is the change not reflected?Please let me know.

(There is no such thing as file disappearing)