docs: Updated for docker cp and its API changes

Documented changes to API to enable new `docker cp` behavior.

Added documentation on `docker cp` usage and behavior.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
Upstream-commit: 7e6f0aaece
Component: cli

components/cli/docs/reference/commandline/cp.md

@@ -11,12 +11,81 @@ weight=1
 
 # cp
 
-    Usage: docker cp CONTAINER:PATH HOSTDIR|-
+Copy files/folders between a container and the local filesystem.
 
-    Copy files/folders from the PATH to the HOSTDIR.
+    Usage:  docker cp [options] CONTAINER:PATH LOCALPATH|-
+            docker cp [options] LOCALPATH|- CONTAINER:PATH
 
-Copy files or folders from a container's filesystem to the directory on the
-host. Use '-' to write the data as a tar file to `STDOUT`. `CONTAINER:PATH` is
-relative to the root of the container's filesystem.
+    --help  Print usage statement
 
+In the first synopsis form, the `docker cp` utility copies the contents of
+`PATH` from the filesystem of `CONTAINER` to the `LOCALPATH` (or stream as
+a tar archive to `STDOUT` if `-` is specified).
 
+In the second synopsis form, the contents of `LOCALPATH` (or a tar archive
+streamed from `STDIN` if `-` is specified) are copied from the local machine to
+`PATH` in the filesystem of `CONTAINER`.
+
+You can copy to or from either a running or stopped container. The `PATH` can
+be a file or directory. The `docker cp` command assumes all `CONTAINER:PATH`
+values are relative to the `/` (root) directory of the container. This means
+supplying the initial forward slash is optional; The command sees
+`compassionate_darwin:/tmp/foo/myfile.txt` and
+`compassionate_darwin:tmp/foo/myfile.txt` as identical. If a `LOCALPATH` value
+is not absolute, is it considered relative to the current working directory.
+
+Behavior is similar to the common Unix utility `cp -a` in that directories are
+copied recursively with permissions preserved if possible. Ownership is set to
+the user and primary group on the receiving end of the transfer. For example,
+files copied to a container will be created with `UID:GID` of the root user.
+Files copied to the local machine will be created with the `UID:GID` of the
+user which invoked the `docker cp` command.
+
+Assuming a path separator of `/`, a first argument of `SRC_PATH` and second
+argument of `DST_PATH`, the behavior is as follows:
+
+- `SRC_PATH` specifies a file
+    - `DST_PATH` does not exist
+        - the file is saved to a file created at `DST_PATH`
+    - `DST_PATH` does not exist and ends with `/`
+        - Error condition: the destination directory must exist.
+    - `DST_PATH` exists and is a file
+        - the destination is overwritten with the contents of the source file
+    - `DST_PATH` exists and is a directory
+        - the file is copied into this directory using the basename from
+          `SRC_PATH`
+- `SRC_PATH` specifies a directory
+    - `DST_PATH` does not exist
+        - `DST_PATH` is created as a directory and the *contents* of the source
+           directory are copied into this directory
+    - `DST_PATH` exists and is a file
+        - Error condition: cannot copy a directory to a file
+    - `DST_PATH` exists and is a directory
+        - `SRC_PATH` does not end with `/.`
+            - the source directory is copied into this directory
+        - `SRC_PAPTH` does end with `/.`
+            - the *content* of the source directory is copied into this
+              directory
+
+The command requires `SRC_PATH` and `DST_PATH` to exist according to the above
+rules. If `SRC_PATH` is local and is a symbolic link, the symbolic link, not
+the target, is copied.
+
+A colon (`:`) is used as a delimiter between `CONTAINER` and `PATH`, but `:`
+could also be in a valid `LOCALPATH`, like `file:name.txt`. This ambiguity is
+resolved by requiring a `LOCALPATH` with a `:` to be made 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.
+
+Using `-` as the first argument in place of a `LOCALPATH` will stream the
+contents of `STDIN` as a tar archive which will be extracted to the `PATH` in
+the filesystem of the destination container. In this case, `PATH` must specify
+a directory.
+
+Using `-` as the second argument in place of a `LOCALPATH` will stream the
+contents of the resource from the source container as a tar archive to
+`STDOUT`.