Create Docker image

[krlmlr]

Expand docker branch.

Installing everything from rcheckserver makes the image prohibitively large.

• [x] Install R from source
• [x] Install sysreqs
• [x] Install tic, using sysreqs from sysreqs
• [x] Only install dependencies if DESCRIPTION changes
• [ ] Minimize image, currently ~400 MB
• [ ] Incremental builds?
  • Tag monthly, only build from scratch once a month (if the monthly image is missing)?
  • https://medium.com/swlh/incremental-docker-builds-for-monolithic-codebases-2dae3ea950e?
• [ ] Push to GitHub registry -- will it be downloaded faster? https://github.com/whoan/docker-build-with-cache-action#github-registry

TBD.