Update Ubuntu 12.04 to 16.04. Version 16.04 is recently EOL but testing on an old version is beneficial.
Update Ubuntu 18.04 to 20.04.
Update Fedora 32 to 33. Version 34 would have been preferred but there were some build issues, i.e. the default shell did not work with configure, and after ksh was installed configure locked up.
Add --no-install-recommends to apt-get commands to save a bit of time and space.
Update test Dockerfile to run in multiple steps. This makes the container larger but also makes rebuilding after changes faster. The --squash option may be used to keep the container small.
Remove obsolete casts in protocol/parallel module. These casts were included in the original migration because Ubuntu 12.04 32-bit gcc required them, but Ubuntu 16.04 32-bit gcc complains. There is no production issue here since at this point in the code the file descriptors are guaranteed to be >= 0.
The tests worked fine on multiple architectures, but would only run "bare metal", i.e. tests that required containers could not be run.
Enable basic multi-architecture support by allowing containers to be built using whatever architecture the host supports. Also allow cached containers to be defined for multiple architectures in container.yaml.
Add a Dockerfile which can be used as a container for other containers to provide a consistent development environment.
The primary goal is to allow development on Mac M1 but other architectures should find these improvements useful.
