Specify host mount prefix with envvar.

closes #1120

plugins/inputs/system/DISK_README.md

@@ -16,6 +16,14 @@ https://en.wikipedia.org/wiki/Df_(Unix) for more details.
   # mount_points = ["/"]
 ```
 
+Additionally, the behavior of resolving the `mount_points` can be configured by using the `HOST_MOUNT_PREFIX` environment variable.
+When present, this variable is prepended to the mountpoints discovered by the plugin before retrieving stats.
+The prefix is stripped from the reported `path` in the measurement.
+This settings is useful when running `telegraf` inside a docker container to report host machine metrics.
+In this case, the host's root volume should be mounted into the container and the `HOST_MOUNT_PREFIX` and `HOST_ETC` environment variables set.
+
+`docker run -v /:/hostfs:ro -e HOST_MOUNT_PREFIX=/hostfs -e HOST_ETC=/hostfs/etc telegraf-docker`
+
 ### Measurements & Fields:
 
 - disk

plugins/inputs/system/ps.go

@@ -81,8 +81,10 @@ func (s *systemPS) DiskUsage(
 				continue
 			}
 		}
-		if _, err := os.Stat(p.Mountpoint); err == nil {
-			du, err := disk.DiskUsage(p.Mountpoint)
+		mountpoint := os.Getenv("HOST_MOUNT_PREFIX") + p.Mountpoint
+		if _, err := os.Stat(mountpoint); err == nil {
+			du, err := disk.DiskUsage(mountpoint)
+			du.Path = p.Mountpoint
 			if err != nil {
 				return nil, err
 			}