Install letsencrypt cer to trust CA in ubuntu

Install ca-certificates

sudo apt-get install ca-certificates

Down load cer from letsencrypt

cd /usr/share/ca-certificates

sudo wget https://letsencrypt.org/certs/isrgrootx1.pem  -O isrgrootx1.crt

sudo wget https://letsencrypt.org/certs/letsencryptauthorityx3.pem  -O letsencryptauthorityx3.crt

Update CA

sudo dpkg-reconfigure ca-certificates