improve Dockerfile documentation
improve Dockerfile documentation

bitemyapp/persistent,jcristovao/persistent,mkscrg/persistent,greydot/persistent,mitchellwrosen/persistent,superduper/persistent,superduper/persistent,pseudonom/persistent,gbwey/persistent,mitchellwrosen/persistent,jcristovao/persistent,jasonzoladz/persistent,meteficha/persistent,junjihashimoto/persistent,psibi/persistent,greydot/persistent,plow-technologies/persistent,superduper/persistent,junjihashimoto/persistent,greydot/persistent,jcristovao/persistent,rdnetto/persistent,gbwey/persistent,mkscrg/persistent,nakaji-dayo/persistent,nakaji-dayo/persistent,meteficha/persistent,pseudonom/persistent,erikd/persistent,naushadh/persistent,yesodweb/persistent,meteficha/persistent,jasonzoladz/persistent,gbwey/persistent,andrewthad/persistent,mkscrg/persistent,mitchellwrosen/persistent,andrewthad/persistent,rdnetto/persistent,paul-rouse/persistent,andrewthad/persistent,jasonzoladz/persistent,nakaji-dayo/persistent,creichert/persistent