Feature request : Add a security policy, add an easy way to track outdated templates

[malko]

As many people will use these templates for self-hosting public facing instance of templates in this repository we should add a security policy to help people report security problems with for example outdated templates or misconfigured ones.

Also, it probably would be nice to have a simple way for maintainers and contributors of this repository to have a script to check for outdated version of templates. This will allow contributors to know which templates require attention and need update in an easy way. It would be even better if this can be run with a github action and published in an md file in this repository.

[Siumauricio]

I think it would be a good idea, only that docker images can come from different places, gcr, dockerhub, etc, it would be a bit complicated to check the latest images or if they are outdated.