document docker staging directory.

website/source/docs/builders/docker.html.md

@@ -354,3 +354,11 @@ the future:
     and other metadata. Packer builds a raw Docker container image that has none
     of this metadata. You can pass in much of this metadata at runtime with
     `docker run`.
+
+## Overriding the host directory
+
+By default, Packer creates a temporary folder under your home directory, and
+uses that to stage files for uploading into the container. If you would like to
+change the path to this temporary folder, you can set the `PACKER_TMP_DIR`
+environment variable. This can be useful, for example, if you have your home
+directory permissions set up to disallow access from the docker daemon.