pipeline: inputs: docker-metrics: add container path config (#1516)

Signed-off-by: Haithem Khelif <haithem.khelif@icloud.com>

Author: hkhelif

pipeline/inputs/docker-metrics.md

@@ -16,6 +16,7 @@ The plugin supports the following configuration parameters:
 | Include      | A space-separated list of containers to include |         |
 | Exclude      | A space-separated list of containers to exclude |         |
 | Threaded | Indicates whether to run this input in its own [thread](../../administration/multithreading.md#inputs). | `false` |
+| path.containers | Used to specify the container directory if Docker is configured with a custom "data-root" directory. | `/var/lib/docker/containers` |
 
 If you set neither `Include` nor `Exclude`, the plugin will try to get metrics from _all_ the running containers.