cleanup, get proper digest into docker provenance

Signed-off-by: Joe Eltgroth <jeltgroth@vmware.com>

.github/actions/srp-helper/publish-container/publish.sh

@@ -28,8 +28,7 @@ observer_agent -m stop_observer -f network_provenance.json
 key="${registry}/${BUILDPACK}-buildpack:${version}"
 echo "key set to ${key}"
 action="publish"
-docker pull "${key}"
-image=$(docker inspect --format='{{index .RepoDigests 0}}' "${key}")
+image=$(docker inspect --format='{{json .RepoDigests}}' "${key}" | jq '.[0]')
 echo "image set to ${image}"
 srp provenance add-output docker \
     	--set-key="${key}" \