A better docker container for development

[cpauvert]

currently, I could not force in the Dockerfile to have {bslib} >= 0.5.0, so I used the docker commit after a manual update 😱. I continued with {rhub} and {rsconnect}. So I need a longer-term strategy:

• could be https://rstudio.github.io/renv/articles/docker.html
• could be packrat seems to have been replaced by {renv}