Pass variables in environment

- Pass secret as SECRET environment variable, instead of requiring
  editing and adding a config file. If none was passed, a generated
  one will be printed and the container will exit.
- Disabled webui, as it would conflict with setting the secret in
  the config file.
- Added instructions on how to link another container to a btsync
  container.
- Automatically set the device name based on the generated host name.
- Moved the btsync data folder to /btsync