Cache intermediate docker layers on travis

[betatim]

Some builds tests take a long time because they have to build our repo2docker "base" again and again.

Travis can cache files between builds however they explicitly mention docker images as something not to cache. I think the reasoning is that pulling them from docker hub is just as fast/slow. In our case we want to cache the intermediate build layers.

What do people think about adding that and how it relates to Travis' wishes about docker image caching?

[minrk]

Worth a shot

[Madhu94]

Is this still relevant ?

[betatim]

Would be interesting to experiment with to find out if this speeds up our CI runs.

[manics]

I'll close this, we've got other issues related to caching and speeding up builds.