Support for custom commands upon entering container

**Is your feature request related to a problem? Please describe.**
Today, for running custom commands when entering a toolbox container the recommendations is to have a custom .bashrc entry, i.e.: [toolbox.sh](https://github.com/josecastillolema/dotfiles/blob/main/bashrc.d/toolbox.sh).
This makes the toolbox configuration split into two files, the Containerfile (i.e.: [Containerfile](https://github.com/josecastillolema/toolbox-images/blob/main/ocaml-5/Containerfile)) and the bash script ^.

**Describe the solution you'd like**
To follow best IaC/declarative practices, it would be better to have all the configuration related to the container in one place, the Containerfile. For that to happen, toolbox should support a way to place custom commands somewhere in the Containerfile (i.e. the `RUN` section).