…chema allows for one

to specify container image, platform, mounts, options.

The container platform support includes docker, podman and singularity.

When running tests in container, the build script is updated to invoke the appropriate container
invocation. The stage directory is mounted in container as /buildtest.

We added an example container configuration file used for declaring few container executors.
Add few example buildspecs for running test in container.

Fix BuildExecutor class to ensure executor are declared properly