add fsleyes

[bcdarwin]

Currently we install FSL via the Ubuntu Focal package, which pulls in FSL 5.0.x. However, fsleyes was not merged into FSL until 6.x, but is not available separately from Ubuntu/NeuroDebian repos as of 20.04 Focal, the version we are currently based on. Thus, we either need to upgrade to FSL 6 or install fsleyes separately. According to repology.org, fsleyes is available from (in addition to downloading and installing by hand) Spack, Gentoo Science Overlay, AUR, and older NeuroDebian releases, while FSL 6 is available only from AUR, Gentoo Science, and Spack. FSL 6 also has a conda-based fsl-installer.py updating the older fslinstaller.py.

It would also suffice to have a separate container for FSL (allowing us to remove it from the main container).