ci: Add docker based test framework

* Add documentation
* Add .dockerignore: minimize docker context size to improve speed
* Add Makefile: to easily run various docker stages
* Add a CMake sample project to check CMake install rules

ci/sample/main.cpp

@@ -0,0 +1,11 @@
+#include <iostream>
+
+#include "cpuinfo_x86.h"
+
+using namespace cpu_features;
+
+int main(int /*argc*/, char** /*argv*/) {
+  static const X86Features features = GetX86Info().features;
+  std::cout << std::endl;
+  return 0;
+}