update docs/reference/commandline/cp.md

Close #22020

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

docs/reference/commandline/cp.md

@@ -81,7 +81,17 @@ you must be explicit with a relative or absolute path, for example:
     `/path/to/file:name.txt` or `./file:name.txt`
 
 It is not possible to copy certain system files such as resources under
-`/proc`, `/sys`, `/dev`, and mounts created by the user in the container.
+`/proc`, `/sys`, `/dev`, [tmpfs](run.md#mount-tmpfs-tmpfs), and mounts created by
+the user in the container. However, you can still copy such files by manually
+running `tar` in `docker exec`. For example (consider `SRC_PATH` and `DEST_PATH`
+are directories):
+
+    $ docker exec foo tar Ccf $(dirname SRC_PATH) - $(basename SRC_PATH) | tar Cxf DEST_PATH -
+
+or
+
+    $ tar Ccf $(dirname SRC_PATH) - $(basename SRC_PATH) | docker exec -i foo tar Cxf DEST_PATH -
+
 
 Using `-` as the `SRC_PATH` streams the contents of `STDIN` as a tar archive.
 The command extracts the content of the tar to the `DEST_PATH` in container's

man/docker-cp.1.md

@@ -78,7 +78,16 @@ you must be explicit with a relative or absolute path, for example:
     `/path/to/file:name.txt` or `./file:name.txt`
 
 It is not possible to copy certain system files such as resources under
-`/proc`, `/sys`, `/dev`, and mounts created by the user in the container.
+`/proc`, `/sys`, `/dev`, tmpfs, and mounts created by the user in the container.
+However, you can still copy such files by manually running `tar` in `docker exec`.
+For example (consider `SRC_PATH` and `DEST_PATH` are directories):
+
+    $ docker exec foo tar Ccf $(dirname SRC_PATH) - $(basename SRC_PATH) | tar Cxf DEST_PATH -
+
+or
+
+    $ tar Ccf $(dirname SRC_PATH) - $(basename SRC_PATH) | docker exec -i foo tar Cxf DEST_PATH -
+
 
 Using `-` as the `SRC_PATH` streams the contents of `STDIN` as a tar archive.
 The command extracts the content of the tar to the `DEST_PATH` in container's