Fix Docker invocation (#300)

* fix syntax error in Dockerfile

* fix Docker invocation

It makes no sense to pass the `-t` argument twice and we need to tell
Docker where to find the Dockerfile. Also, it makes sense to write the
config file in a standard location. In my tests, I also had to create
the directory by hand otherwise I got this error:

    cdparanoia can defeat the audio cache on this drive.
    Adding drive cache behaviour to configuration file.
    Traceback (most recent call last):
      File "/usr/local/bin/whipper", line 11, in <module>
        load_entry_point('whipper==0.7.0', 'console_scripts', 'whipper')()
      File "/usr/local/lib/python2.7/dist-packages/whipper-0.7.0-py2.7.egg/whipper/command/main.py", line 36, in main
        ret = cmd.do()
      File "/usr/local/lib/python2.7/dist-packages/whipper-0.7.0-py2.7.egg/whipper/command/basecommand.py", line 139, in do
        return self.cmd.do()
      File "/usr/local/lib/python2.7/dist-packages/whipper-0.7.0-py2.7.egg/whipper/command/basecommand.py", line 139, in do
        return self.cmd.do()
      File "/usr/local/lib/python2.7/dist-packages/whipper-0.7.0-py2.7.egg/whipper/command/drive.py", line 63, in do
        info[0], info[1], info[2], t.defeatsCache)
      File "/usr/local/lib/python2.7/dist-packages/whipper-0.7.0-py2.7.egg/whipper/common/config.py", line 115, in setDefeatsCache
        section = self._findOrCreateDriveSection(vendor, model, release)
      File "/usr/local/lib/python2.7/dist-packages/whipper-0.7.0-py2.7.egg/whipper/common/config.py", line 164, in _findOrCreateDriveSection
        self.write()
      File "/usr/local/lib/python2.7/dist-packages/whipper-0.7.0-py2.7.egg/whipper/common/config.py", line 58, in write
        shutil.move(path, self._path)
      File "/usr/lib/python2.7/shutil.py", line 316, in move
        copy2(src, real_dst)
      File "/usr/lib/python2.7/shutil.py", line 144, in copy2
        copyfile(src, dst)
      File "/usr/lib/python2.7/shutil.py", line 97, in copyfile
        with open(dst, 'wb') as fdst:
    IOError: [Errno 13] Permission denied: u'/home/worker/.config/whipper/whipper.conf'

* remove needless tag version

:latest is implicit so it's useless noise as well.

* Update README.md

Dockerfile

@@ -34,7 +34,7 @@ RUN mkdir /whipper
 COPY . /whipper/
 RUN cd /whipper/src && make && make install \
   && cd /whipper && python2 setup.py install \
-  && rm -rf /whipper
+  && rm -rf /whipper \
   && whipper -v
 
 # add user