Selective session info

[nuest]

When creating a Dockerfile from sessionInfo(), it currently lists all packages from the session info, including the ones that are loaded via namespace and not attached. This is great as it is explicit and complete, but it can also quickly fill the Dockerfile with information and configuration that might be unintended by or confusing for a user.

• [ ] add option to Dockerfile creation to only install the attached packages
• [ ] evaluate further how packages are attached via namespace
• [ ] check that even when not explicitly installing the loaded packages, they are still in the container (because the explicitly attached packages list them as required/suggested (is there a difference here?))