Upgrade Docker image base

[mgxd]

Ubuntu 16.04 End of Life (EOL) is scheduled for April 30, 2021.

We should probably bump up to 20.04

EDIT: They've added an extended security maintenance until 2024 - https://ubuntu.com/security/esm

[oesteban]

Yes, that will force us to change a fair amount of stuff. Probably, drop neurodebian (afni will not be available and we may have to take the leap and jump to FSL6 at last).