ci: improve docker workflow to push latest tag on release

Changes

if a release triggered the docker workflow, also push the image to the 'latest' tag
allow setting the R version from the github actions workflow

(~Strikethrough~ any points that are not applicable.)

[x] This comment contains a description of changes with justifications, with any relevant issues linked.
~[ ] Write unit tests for any new features, bug fixes, or other code changes.~
~[ ] Update the docs if there are any API changes (roxygen comments, vignettes, readme, etc.).~
~[ ] Update NEWS.md with a short description of any user-facing changes and reference the PR number. Guidelines: https://keepachangelog.com/en/1.1.0/~
~[ ] Run devtools::check() locally and fix all notes, warnings, and errors.~