PMM Server Docker image: Remove yum cache to reduce size of image

Problem

The docker image has ~150MB of data from yum packages that could  be removed and thus would reduce the image size.

[[email protected] opt]# du -sh /var/cache/yum/         
159M	/var/cache/yum/ 

Suggestion

yum clean all 

How to test

The size of the final image should be ~1.7GB. You can check it from console:

perconalab/pmm-server-fb             PR-1845-91cd351                                         d5e42aa2c330   3 hours ago     1.7GB
percona/pmm-server                   2.16                                                    03e3d43682dd   8 weeks ago     1.88GB

 Or from DockerHub (It displays the already compressed size. It should be ~529.54MB).

The size can be slightly higher or lower, as we will have other changes.

It is also a good idea to check that the button updates are working correctly.