Differentiating dependency build issues, dependency API changes, and package bugs for Docker

[TheChymera]

I mentioned briefly how this is a feature which our system could easily offer, and which the binary package management via docker, which is commonly used for TravisCI cannot.

Travis however has a number of different phases which we might be able to use to break up the docker image selection process into conceptually coherent bits. These sections can be seen e.g. in .travis.yml:

• before_install
• install
• scripts