Description of problem:
I find several differences in behaviour of --installroot between yum and dnf. But it does not make sense to me to report the unless there is clear documentation of this option and what behaviour is correct.

First: documentation in dnf is very short:
       --installroot=<path>
              set install root
which I find as regression from yum documentation:
       --installroot=root
              Specifies an alternative installroot, relative to which all packages will be installed. Think of this like doing "chroot  <root>
              yum"  except  using  --installroot  allows  yum to work before the chroot is created.  Note: You may also want to use the option
              --releasever=/ when creating the installroot as otherwise the $releasever value is taken from the rpmdb within  the  installroot
              (and thus. will be empty, before creation).
              Configuration Option: installroot

But even the yum expanded documentation does not state, how installroot relate to yum.repos.d/* files. Are they taken from host of from installroot? Log files are placed to host /var/log or to installroot? 

Version-Release number of selected component (if applicable):
dnf-0.6.2-1.fc21.noarch