[travis-ci] How to run travis-ci locally

Travis-ci offers a new container-based infrastructure that uses docker. This can be very useful if you're trying to troubleshoot a travis-ci build by reproducing it locally. This is taken from Travis CI's documentation.

Troubleshooting Locally in a Docker Image

If you're having trouble tracking down the exact problem in a build it often helps to run the build locally. To do this you need to be using our container based infrastructure (ie, have sudo: false in your .travis.yml), and to know which Docker image you are using on Travis CI.

Running a Container Based Docker Image Locally

1. Download and install the Docker Engine.

2. Select an image from Docker Hub. If you're not using a language-specific image pick ci-ruby. Open a terminal and start an interactive Docker session using the image URL:

   docker run -it travisci/ubuntu-ruby:18.04 /bin/bash
   

3. Switch to the travis user:

   su - travis
   

4. Clone your git repository into the / folder of the image.
5. Manually install any dependencies.
6. Manually run your Travis CI build command.