Use the HTTP Last-Modified http header as the mtime value for ADD cmd when present
Closes
@@ -376,7 +376,11 @@ destination container.
All new files and directories are created with a UID and GID of 0.
In the case where `<src>` is a remote file URL, the destination will
-have permissions of 600.
+have permissions of 600. If the remote file being retrieved has an HTTP
+`Last-Modified` header, the timestamp from that header will be used
+to set the `mtime` on the destination file. Then, like any other file
+processed during an `ADD`, `mtime` will be included in the determination
+of whether or not the file has changed and the cache should be updated.
> **Note**:
> If you build by passing a `Dockerfile` through STDIN (`docker
