readme: add note on changing security context of mounted volumes with selinux
readme: add note on changing security context of mounted volumes with selinux

youhong316/docker-gitlab-ci,bboehmke/docker-gitlab-ci,bboehmke/docker-gitlab-ci,adiferd/docker-gitlab-ci,adiferd/docker-gitlab-ci,outcoldman/docker-gitlab-ci,sameersbn/docker-gitlab-ci,outcoldman/docker-gitlab-ci,youhong316/docker-gitlab-ci,sameersbn/docker-gitlab-ci