Add a HEALTHCHECK to the dockerfile so automated scripts can correctly wait until files are downloaded

[JamesXNelson]

We should add a healthcheck script which verifies that all files have been downloaded and set that as the Dockerfile HEALTHCHECK. Without this, demo deployments can't tell if pulling examples failed or simply did not complete yet.

If the main download script touches a before/after marker file, we can make the HEALTHCHECK simply look for the after marker file to know that all data was successfully downloaded.