shell-tools: fix inaccessible pushImageOrManifest

The function has to be exported and used with bash -c here in order to
work with xargs.

See for more info:
https://stackoverflow.com/questions/11003418/calling-shell-functions-with-xargs

resources/shell-tools/bin/push.sh

@@ -73,5 +73,6 @@ if [ "$CONTAINER_IMAGE_NAME" = "invalid" ]; then
     printUsage
 fi
 
+export -f pushImageOrManifest
 # shellcheck disable=SC2086
-$CONTAINER_CMD images --format "{{.Repository}}:{{.Tag}}" "$CONTAINER_IMAGE_NAME" | grep "$CONTAINER_IMAGE_NAME" | xargs -L 1 pushImageOrManifest
+$CONTAINER_CMD images --format "{{.Repository}}:{{.Tag}}" "$CONTAINER_IMAGE_NAME" | grep "$CONTAINER_IMAGE_NAME" | xargs -L 1 -I {} bash -c 'pushImageOrManifest "$@"' _ {}