use_docker_packages: Add dependencies to Dockerfile
In jimbrig/jimstemplates: Collection of templates

Adds package dependencies as a new RUN statement to Dockerfile. Sorts packages first into source (cran & github) and then alphabetically.

use_docker_packages(
  packages,
  github = NULL,
  strict = TRUE,
  file = "Dockerfile",
  write = TRUE,
  open = write,
  append = TRUE
)

`packages`	Which packages to add.
`github`	Are there github packages?
`strict`	Defaults to TRUE, force a specific version for github packages.
`file`	Where is the 'Dockerfile'?
`write`	Should the 'Dockerfile' be modified?
`open`	Should the file be opened?
`append`	Should the return value be appended to the 'Dockerfile'?