Support manual installation of required libraries

[nuest]

sysreqs might not provide all required information, so advanced users should be able to provide installation instructions somehow.

Users could simply do this within their R Markdown document with system() calls, but that's not very nice, and while transparent might lead to people hiding these functions etc.

Ideas

• [ ] support an install.R file, comparable to rocker/binder (automatically add a COPY statement if file is found, RUN it in the Dockerfile)
• [ ] check apt.txt approach (or discussions) in binder
• [ ] add a wrapper for RUN statements to be added to dockerfile(...)
• [ ] ~~parse and "extend" existing Dockerfile~~ error prone, complicated