While installing Docker on Ubuntu 14.04.3 LTS getting error signatures couldn't be verified

We are trying to install docker on Ubuntu 14.04.3 LTS (GNU/Linux 3.19.0-25-generic x86_64)

In the first step : sudo apt-get update we are getting error below:
The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY

Kindly advise; how can we resolve this error?