Ensure calls to `ldconfig` are executed towards the end of `build` specifications

[shnizzedy]

Number 7 in Singularity's Support for Docker and OCI: Best Practices is

Ensure calls to ldconfig are executed towards the end of build specifications (e.g., Dockerfile), so that the library cache is updated when the container is created.

As far as I know, neurodocker doesn't do that automatically (except for in the AFNI installation), so I just include --run 'ldconfig' as the final flag when I run neurodocker

Originally posted by @shnizzedy in https://github.com/ReproNim/neurodocker/issues/321#issuecomment-812644282