docs/docker.rst: updating supported ubuntu version
docs/docker.rst: updating supported ubuntu version

Also linking to docker documentation on installing it on ubuntu LTS

4eek/tsuru,gciotta/tsuru,firebitsbr/tsuru,tuxmonteiro/tsuru,kgrvamsi/tsuru,guilhermebr/tsuru,mderaldo/tsuru,RichardKnop/tsuru,jblancett/tsuru,gciotta/tsuru,kgrvamsi/tsuru,mderaldo/tsuru,jblancett/tsuru,cgvarela/tsuru,tuxmonteiro/tsuru,jblancett/tsuru,4eek/tsuru,ggarnier/tsuru,RichardKnop/tsuru,firebitsbr/tsuru,ggarnier/tsuru,ArxdSilva/tsuru,kgrvamsi/tsuru,tuxmonteiro/tsuru,tsuru/tsuru,hcxiong/tsuru,guilhermebr/tsuru,ggarnier/tsuru,gciotta/tsuru,tsuru/tsuru,gwmoura/tsuru,hugoantunes/tsuru,hugoantunes/tsuru,ArxdSilva/tsuru,4eek/tsuru,hcxiong/tsuru,tsuru/tsuru,guilhermebr/tsuru,cezarsa/tsuru,ArxdSilva/tsuru,Endika/tsuru,Endika/tsuru,RichardKnop/tsuru,ggarnier/tsuru,gwmoura/tsuru,gwmoura/tsuru,cgvarela/tsuru,cgvarela/tsuru,cezarsa/tsuru,hcxiong/tsuru,cezarsa/tsuru,guilhermebr/tsuru,firebitsbr/tsuru,cezarsa/tsuru,Endika/tsuru,mderaldo/tsuru,tsuru/tsuru,ArxdSilva/tsuru