Start as root

for some reasons (describe below), I need to test as root user dropbox, is there a simple way to do that?
### Reasons:
- I have started a container with `--volumes-from=another_data_container` on **host A**.
- another_data_container was writing file as uid 1000 gid 1000.
- I have added a group with gid 1000 in dropbox, and added dropbox user into the group.
- Into `another_data_container`, I have updated folders permissions using `find . -type d -exec chmod g+rwx` and files permissions usings `find . -type f -exec chmod g+rw`
- I linked my dropbox account
- file got synced to dropbox (without adding group permissions, dropbox wasn't allowed to upload those files)
- I have started a container with `--volumes-from=another_data_container` on **host B**
- Dropbox container from **host B** should download `another_data_container` files but I have the following error :

```
dropbox status
Syncing (34 files remaining)
Downloading 34 files...
Can't sync "myfile.txt" (permission denied)
```

Permissions of this file as been uploaded to dropbox with `UID 1000/GID 1000` so for me it should work.

Start with the container setup using --volumes-from and the dropbox status output, then compare the shared files' UID/GID and group permissions with the Dropbox process identity. There is no source file or test named; done should establish and document a supported way to sync these files, including whether starting Dropbox as root resolves the permission-denied result.

Written by the indexing model from the issue text.