Dockerfile missing six package install

When trying to build a clean Docker image from the source code, the following error happens when running:

```shell
$ git clone --depth=1 --recursive https://github.com/HDFGroup/h5serv.git
...
$ cd h5serv
$ docker build --tag my-h5serv .
...
$ docker run -v /tmp/h5serv-data:/data -p 5000:5000 my-h5serv
Traceback (most recent call last):
File "/usr/local/lib/python3.6/runpy.py", line 193, in _run_module_as_main
"__main__", mod_spec)
File "/usr/local/lib/python3.6/runpy.py", line 85, in _run_code
exec(code, run_globals)
File "h5serv/__main__.py", line 6, in
from h5serv import app
File "/usr/local/src/h5serv/h5serv/app.py", line 13, in
import six
ModuleNotFoundError: No module named 'six'
```