title: .dockerignore mentioned files are not ignored
body: <p>I'm having in <code>Dockerfile</code>:</p>
  
  <pre><code>ENV DATARATOR_HOME /usr/local/share/datarator
  RUN mkdir -p $DATARATOR_HOME
  COPY . $DATARATOR_HOME
  </code></pre>
  
  <p>and <code>.dockerignore</code> file:</p>
  
  <pre><code>/Gemfile.lock
  /coverage
  /spec
  *.bundle
  *.so
  *.o
  *.a
  mkmf.log
  *.swp
  /.*
  /tmp
  /log
  </code></pre>
  
  <p>However, once showing files in the built container, I can see also those that are supposed to be ignored:</p>
  
  <pre><code>/usr/local/share/datarator # ls -lha
  total 128
  drwxr-xr-x   10 root     root        4.0K Mar 29 21:01 .
  drwxr-xr-x    4 root     root        4.0K Mar 29 21:00 ..
  drwxr-xr-x    2 root     root        4.0K Mar 29 21:01 .bundle
  -rw-rw-r--    1 root     root          24 Mar 29 20:37 .coveralls.yml
  -rw-rw-r--    1 root     root          81 Mar 29 20:37 .dockerignore
  drwxrwxr-x    8 root     root        4.0K Mar 29 20:37 .git
  -rw-rw-r--    1 root     root          85 Mar 29 20:37 .gitignore
  -rw-rw-r--    1 root     root        1.2K Mar 29 20:37 .travis.yml
  -rw-rw-r--    1 root     root         509 Mar 29 20:37 .vimrc
  -rw-rw-r--    1 root     root         959 Mar 29 20:37 Dockerfile
  -rw-rw-r--    1 root     root          94 Mar 29 20:37 Gemfile
  -rw-r--r--    1 root     root        2.7K Mar 29 21:01 Gemfile.lock
  -rw-rw-r--    1 root     root         343 Mar 29 20:37 Guardfile
  -rw-rw-r--    1 root     root        1.0K Mar 29 20:37 LICENSE.txt
  -rw-rw-r--    1 root     root          71 Mar 29 20:37 Procfile
  -rw-rw-r--    1 root     root       14.8K Mar 29 20:37 README.md
  -rw-rw-r--    1 root     root         198 Mar 29 20:37 Rakefile
  drwxrwxr-x    2 root     root        4.0K Mar 29 20:37 bin
  drwxrwxr-x    2 root     root        4.0K Mar 29 20:37 config
  -rw-rw-r--    1 root     root          97 Mar 29 20:37 config.ru
  -rw-r--r--    1 root     root       16.0K Mar 29 21:01 datarator-0.0.1.gem
  -rw-rw-r--    1 root     root        1.7K Mar 29 20:37 datarator.gemspec
  drwxrwxr-x    4 root     root        4.0K Mar 29 20:37 lib
  drwxrwxr-x    2 root     root        4.0K Mar 29 20:37 log
  drwxrwxr-x    3 root     root        4.0K Mar 29 20:37 spec
  drwxrwxr-x    2 root     root        4.0K Mar 29 20:37 tmp
  </code></pre>
  
  <p>How can I achieve having all those mentioned in the <code>.dockerignore</code> file ignored?</p>