Enable SPDX document validation for CI

[nishakm]

Description
As development increases on the project, it is getting hard to maintain the generation of valid SPDX documents for the container images. So enable CI checks for SPDX if anything in the generator or formatting code changes.

To Do

1. Setup requires install of openjdk and the SPDX tools from their release page: https://github.com/spdx/tools/releases
2. Then run tern report -f spdxtagvalue -i <a chosen image> -o spdx.txt and run the validation tool on the output.

[rnjudge]

The new SPDX validation tool is available here: https://github.com/spdx/tools-java