Description of problem:
Traceback after running 'dnf install vim'

Version-Release number of selected component (if applicable):
dnf-2.6.3

How reproducible:
always

Steps to Reproduce:
1. docker run -it --user 1000 fedora:26 bash
2. dnf install vim
3.

Actual results:
Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/dnf/cli/main.py", line 64, in main
    return _main(base, args, cli_class, option_parser_class)
  File "/usr/lib/python3.6/site-packages/dnf/cli/main.py", line 95, in _main
    cli.configure(list(map(ucd, args)), option_parser())
  File "/usr/lib/python3.6/site-packages/dnf/cli/cli.py", line 816, in configure
    self._read_conf_file(opts.releasever)
  File "/usr/lib/python3.6/site-packages/dnf/cli/cli.py", line 922, in _read_conf_file
    conf.prepend_installroot(opt)
  File "/usr/lib/python3.6/site-packages/dnf/conf/config.py", line 819, in prepend_installroot
    new_path = self._prepend_installroot_path(opt._get())
  File "/usr/lib/python3.6/site-packages/dnf/conf/config.py", line 823, in _prepend_installroot_path
    root_path = os.path.join(self._get_value('installroot'), path.lstrip('/'))
AttributeError: 'NoneType' object has no attribute 'lstrip'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/bin/dnf", line 58, in <module>
    main.user_main(sys.argv[1:], exit_code=True)
  File "/usr/lib/python3.6/site-packages/dnf/cli/main.py", line 179, in user_main
    errcode = main(args)
  File "/usr/lib/python3.6/site-packages/dnf/cli/main.py", line 64, in main
    return _main(base, args, cli_class, option_parser_class)
  File "/usr/lib/python3.6/site-packages/dnf/base.py", line 106, in __exit__
    self.close()
  File "/usr/lib/python3.6/site-packages/dnf/base.py", line 368, in close
    self.conf.cachedir)
  File "/usr/lib/python3.6/site-packages/dnf/persistor.py", line 483, in __init__
    self.db_path = os.path.join(cachedir, "tempfiles.json")
  File "/usr/lib64/python3.6/posixpath.py", line 78, in join
    a = os.fspath(a)
TypeError: expected str, bytes or os.PathLike object, not NoneType

Expected results:
no traceback

Additional info: