consider caching travis_ci/Dockerfile builds in CI

[cosmicexplorer]

When working on #7064, we added python 3.6.8 to the centos6 base image via pyenv. This was great and the right thing to do (otherwise CentOS makes it quite hard to have multiple versions of python available, which is something we want for the base image). However, it was described to me that building build-support/docker/travis_ci/ can actually take some nontrivial period of time (just building the image itself, before we even start building pants). From a really quick investigation, it seems like it might be possible to cache the docker builds on travis (see especially the second link), but I'm not familiar enough with docker or travis's implementation to know whether there are roadblocks I'm missing.

Some other inspiration for this came from #7065, which allows locally iterating on the CentOS6 image, but that's a bit of a dual problem.

[Eric-Arellano]

I don't think this would make a ton of sense because it increases the complexity of an already very complex CI. We already host our Docker images in Dockerhub.

The cost is only when you build a Docker image locally, and when you do that you'll benefit from Docker's caching.