missing rake command in Dockerfile

[danoe]

Pull Request (PR) description

Dockerfile failed to build as it was missing rake inside the bundle exec

This Pull Request (PR) fixes the following issues

Dockerfile build failed with following error:

Step 10/11 : RUN bundle exec release_checks
 ---> Running in ea81d27199df
bundler: command not found: release_checks
Install missing gem executables with `bundle install`
The command '/bin/sh -c bundle exec release_checks' returned a non-zero code: 127```

[baurmatt]

Tests should be fixed with #519. Could you please rebase your branch? :)

[baurmatt]

Thank you for you contribute! :) This was already fixed in the master branch by #520. I therefore closing this issue.