I wonder if we hit a query string limitation and should instead switch
to post body here. It's the `curl -G -X POST` part making this a POST
with a long query string.
(cherry picked from commit 7b28152a92)
- Use sha256 for build file hash
- Use build file hash as part of the data for the HMAC digest
- Remove a few unnecessary lines from the centos8 dockerfile
- Pass timestamp in UTC