Dockerize: version 'NA' is invalid for package

[ColinFay]

Upon using golem::add_dockerfile() and running a docker build, I came up with RUN Rscript -e 'remotes::install_version("sf",upgrade="never", version = "NA")' in the dockerfile. However, by having a look at the remotes::install_version doc, it seems "NA" is not a valid option for the "version" argument (either a version string, or NULL), and this is not specific to {sf} package.

For now, I simply removed the version and will fix it by hand later (not an emergency).

Migrated from https://github.com/ThinkR-open/golem/issues/616

[ColinFay]

Original issue from @earnaud