Specify Ubuntu base for custom images

[kwabenantim]

Summary

• Specify Ubuntu base for custom images
• Build portability-dev on Ubuntu resolute
• Update portability testing docs, add how to change Ubuntu base, where to find patches etc.

[kwabenantim]

Regarding patching, this requires adding functionality to enable scripts to select appropriate patches depending on factors including (but not limited to):

• Package version (this is currently in place)
• Ubuntu version (this needs adding)
• Compiler version (possibly consider, but can probably assume default Ubuntu system version)
• Python version (possibly consider, but can probably assume default Ubuntu system version)