[Docs] Add Documentation on Pre-Built Container Applications of `ntia-conformance-checker`

[John Speed Meyers]

There is a now containerized version of ntia-conformance-checker available on Docker and on Chainguard's container registry. [Full disclosure: I work at Chainguard.]

I wonder if creating a short section about the existence and use of these containers in the README would be helpful to users?

From Chainguard's cgr.dev registry:

docker pull cgr.dev/chainguard/ntia-conformance-checker:latest

From Docker Hub (Chainguard is a member of the Docker Verified Program):

docker pull chainguard/ntia-conformance-checker:latest 

[Gary O'Neall]

I think this would be helpful to add.

@kestewart - any concerns since it would point to a non-LF website for the container?