Figure out system requirements for Dockerimage

[eliocamp]

Some R packages depend on system requirements which have to be installed before anything. use_dockerfile() could try to figure out which are the required system dependencies based on the dependencies listed in DESCRIPTION and using either sysreqsdb or r-system-requirements.

An alternative would be to have a use_requirements() (or better named) function that does the figuring out and then tells the user the command that they should put on their Dockerfile.