Explore an init / entrypoint directory

[colearendt]

For the postgres docker image, they have the notion of an "init" directory that you can put scripts into to be executed.

https://github.com/docker-library/postgres/blob/0d0485cb02e526f5a240b7740b46c35404aaf13f/12/docker-entrypoint.sh#L128

I think having such a directory (for RStudio Server Pro, at least) may be helpful for making the images more extensible. It also may be another way of starting the launcher 🤷‍♂(Related to #9)

The use case I have presently is I want to symlink the /etc/rstudio/load-balancer file to somewhere else (workaround for this: https://github.com/rstudio/rstudio-pro/issues/1380), and extending our entrypoint script is challenging.

[colearendt]

Holding for #114 since we do not want to conflict

[colearendt]

#337 is a proposed implementation