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.

CONTRIBUTING.md

@@ -0,0 +1,29 @@
+# Contributing
+All contributions are welcome.
+
+There are different ways in which you can contribute to Tracks.
+
+## Documentation
+You can contribute documentation in the [wiki](https://github.com/TracksApp/tracks/wiki).
+
+For example:
+- tutorials on installing Tracks in various environments
+- tutorials on using Tracks (user manual)
+
+## Issues
+If you think having found a problem with Tracks, first search in the [existing issues]((https://github.com/TracksApp/tracks/issues). If you cannot find it, open a new issue and try providing information on your setup and what steps are needed to reproduce the problem.
+
+## Enhancements
+It would be great to first discuss them on the [mailing list](https://groups.google.com/group/TracksApp) so you can figure out if it could be merged or not. You may use the wiki too to describe your change if it is too big for an email.
+
+If you want to contribute an enhancement or a fix, you can:
+
+1. [fork the project](https://help.github.com/articles/fork-a-repo) 
+1. [create a topic branch](http://learn.github.com/p/branching.html).
+1. setup the project with [docker-compose](https://docs.docker.com/compose/) by running `docker-compose up`
+1. then run `./bin/setup` to setup the first run
+1. if you need, you can launch a Rails console with `./bin/rails c` (will run inside Docker)
+1. make your changes and adds/updates relevant tests.
+1. run the test suite with `./bin/rake test` (will run inside Docker)
+1. commit the changes
+1. send a pull request.