Add all dependencies to Dockerfile, even transitive ones

[nuest]

Potentially useful functions:

• Using pkggraph we could explicitly list all R package dependencies of the loaded packages in the Dockerfile. See https://cran.r-project.org/web/packages/pkggraph/vignettes/vignette_pkggraph.html
• tools package could also already do the job:
  • https://www.rdocumentation.org/packages/tools/versions/3.4.1/topics/package_dependencies
• miniCRAN package: https://cran.r-project.org/web/packages/miniCRAN/vignettes/miniCRAN-dependency-graph.html
  • https://www.rdocumentation.org/packages/miniCRAN/versions/0.2.10/topics/pkgDep

This should be controllable by an argument in the dockerfile() function.

[nuest]

For the o2r reproducibility service, this might add considerable overhead because things are installed despite them being available in the base image.