Update Readme.md to fix #4904 with respect to bundler does not match the version in Gemfile.lock

README.md

@@ -158,7 +158,7 @@ internet connection available.
 2. Create a Docker container
    ```sh
    docker create --name middleman -v "$PWD/www-gitlab-com":/site -w /site -p 4567:4567 \
-     -e LC_ALL=C.UTF-8 ruby /bin/bash -c 'bundle install && bundle exec middleman'
+     -e LC_ALL=C.UTF-8 ruby /bin/bash -c 'gem install bundler && bundle install && bundle exec middleman'
    ```
 3. Start the container