Update journald.md to add info about image name

RohitK89 · web-flow · commit 9843b0878113 · 2018-10-12T18:16:35.000-04:00
Related to PR being opened in the `moby` project to add this functionality.
diff --git a/config/containers/logging/journald.md b/config/containers/logging/journald.md
@@ -22,6 +22,7 @@ stores the following metadata in the journal with each message:
 | `CONTAINER_NAME`                     | The container name at the time it was started. If you use `docker rename` to rename a container, the new name is not reflected in the journal entries. |
 | `CONTAINER_TAG`, `SYSLOG_IDENTIFIER` | The container tag ([log tag option documentation](log_tags.md)).                                                                                       |
 | `CONTAINER_PARTIAL_MESSAGE`          | A field that flags log integrity. Improve logging of long log lines.                                                                                   |
+| `IMAGE_NAME`                         | The image name and tag for the container, displayed as `image:tag`. Note that tag will not be displayed if its value is `latest`.                      |
 
 ## Usage
 
