Fix Docker setup

This commit fixes the Docker setup and also the CI build.

Also the contributing guide is updated with the new information related
to Docker. The CONTRIBUTING.md file is used to comply with GitHub
guidelines to help other developers to contribute.

doc/README_DEVELOPERS

@@ -1,43 +0,0 @@
-1. Resources
-
-Tracks is using
-* GitHub to host the git repository, manage git pull requests, and manage bugs and enhancement requests.
-* the mailing list to discuss features and development and interact with users
-
-See README for links to the respective sites
-
-Also see the Development pages on the wiki for details on installing, testing,
-upgrading, etc.
-
-2. Dependencies
-
-The dependencies for Tracks are maintained using bundler. Before starting your
-tracks instance, you'll need to run 'bundle install' to fetch all the
-dependencies
-
-3. Testing
-
-There are some pointers for setting up your Tracks copy for testing at
-https://github.com/TracksApp/tracks/wiki/Testing/
-
-By default, tests are configured to run using sqlite3 in memory mode to
-increase speed. You will need the sqlite3-ruby gem for this.
-
-To avoid showing the migrations as tests are run, add the following to your
-database.yml below 'database: ":memory:"':
-
-  verbosity: quiet
-
-If you want to run tests using another database, that's fine, too. Just change
-your database.yml accordingly.
-
-Running cucumber/selenium tests in :memory mode does not seem to work.
-
-The rspec tests are not actively maintained.
-
-See the wiki for more information on testing:
-https://github.com/TracksApp/tracks/wiki/Testing
-
-4. Contributing
-
-See https://github.com/TracksApp/tracks/wiki/How-to-contribute