Consider Docker to Reduce CI Overhead

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.

Start with the Linux CI job matrix and its repeated apt and cmake setup described in the issue. Compare dependency-installation time with build and test time, then determine how a Docker-based environment could cover every matrix job while preserving the existing header build and test behavior.

Written by the indexing model from the issue text.