[CI] Reduce logging level when checking if docker image exists (#17221)

Prior to this commit, the `image_exists` utility in
`determine_docker_images.py` logged the full response for success, and
the full HTTP error if an exception is caught.  However, this is the
expected behavior when loading a docker image from `tlcpackstaging`,
such as the current images tagged with `20240428-060115-0b09ed018`.
Logging this fallback as an error makes it difficult to find the first
actual error that occurred in CI.

This commit updates these logging statments `logging.info` and
`logging.exception` to instead use `logging.debug`.

Author: Lunderberg

ci/scripts/jenkins/determine_docker_images.py

@@ -62,11 +62,11 @@ def image_exists(spec: str) -> bool:
     name, tag = spec.split(":")
     try:
         r = docker_api(f"repositories/{name}/tags/{tag}")
-        logging.info(f"Image exists, got response: {json.dumps(r, indent=2)}")
+        logging.debug(f"Image exists, got response: {json.dumps(r, indent=2)}")
         return True
     except urllib.error.HTTPError as e:
         # Image was not found
-        logging.exception(e)
+        logging.debug(e)
         return False