coala: Add Travis testing of rultor docker image
coala uses rultor for managing pull requests. rultor has a docker image.
Rultor provides a Dockerfile that is not built and tested on Travis, which limits the ability of newcomers to make improvements and test them to avoid regressions.
coala has a repository rultor-python which extends the yegor256/rultor
docker image to add extra Python dependencies that are missing from the yegor256/rultor docker image.
This task is to add Travis CI testing of the Dockerfile and Docker image to the yegor256/rultor repository.
This task should not substantially revise the Dockerfile. In addition to testing the existing Dockerfile, additional effort should focus on testing it for the types of changes which are pending.
Submit a pull request, and include a comment on the PR linking to your successful Travis build.