X509 : certificate issue with ubuntu 16.04

Hi All,
I am new Linux,I installed Ubuntu in VirtualBox in my windows,I initially had certificate issue which got resolved once i imported the cert from Windows.
Now I am able to access the internet from ubuntu

I installed docker following the below documentation
https://docs.docker.com/engine/insta.../linux/ubuntu/

Now I am getting the below certificate issue

sudo docker pull jenkins
sudo: unable to resolve host
Using default tag: latest
Pulling repository docker.io/library/jenkins
Error while pulling image: Get https://index.docker.io/v1/repositor...enkins/images: x509: certificate signed by unknown authority

curl on index.docker.io
$ curl https://index.docker.io
curl: (60) server certificate verification failed. CAfile: /etc/ssl/certs/ca-certificates.crt CRLfile: none
More details here: http://curl.haxx.se/docs/sslcerts.html

$ curl https://www.google.com
curl: (60) server certificate verification failed. CAfile: /etc/ssl/certs/ca-certificates.crt CRLfile: none
More details here: http://curl.haxx.se/docs/sslcerts.html



How do i resolve the certificate issue.I followed documentation to generate a new self signed certificate with no luck.