In some markdown files we already have the date frontmatter field defined, if
that's the case then we want to tell the update-lastmod script to not overwrite
it with the date that the file got added to the repository as in some cases
(for example the CVEs) the content may be older than when we added it to the
repo.

Build docker container
Ran docker container locally
Verified that the contents of both blog post and advisories are ordered correctly