Consider Docker to Reduce CI Overhead

[alycm]

In https://github.com/KhronosGroup/OpenCL-Headers/issues/142 @oddhack says

N.b. you might consider building a Docker image containing all the right components for this. Even just updating the apt database and installing components from cmake 96 times (# of jobs in the Linux job matrix) takes considerable time.

Sound sensible, I imagine that the test matrix spends far more time on apt that actually building/testing the headers themselves.

[oddhack]

BTW we have a repo for Khronos Dockerfiles in https://github.com/KhronosGroup/DockerContainers - can just add yours and use Ryan's build-and-push script to get the image into dockerhub.