Description of problem:
Traceback after running 'dnf --version'

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 --version
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 804, in configure
    self.base.output)
  File "/usr/lib/python3.6/site-packages/dnf/cli/cli.py", line 112, in print_versions
    rpmdb_sack = dnf.sack._rpmdb_sack(base)
  File "/usr/lib/python3.6/site-packages/dnf/sack.py", line 107, in _rpmdb_sack
    sack = _build_sack(base)
  File "/usr/lib/python3.6/site-packages/dnf/sack.py", line 99, in _build_sack
    dnf.util.ensure_dir(cachedir)
  File "/usr/lib/python3.6/site-packages/dnf/util.py", line 120, in ensure_dir
    os.makedirs(dname, mode=0o755)
  File "/usr/lib64/python3.6/os.py", line 205, in makedirs
    head, tail = path.split(name)
  File "/usr/lib64/python3.6/posixpath.py", line 105, in split
    p = os.fspath(p)
TypeError: expected str, bytes or os.PathLike object, not NoneType

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:
version of dnf
no traceback

Additional info: