--runtime auto?

***In GitLab by @danrue on Nov 16, 2020, 18:54 GMT:***

TuxMake supports --runtimes docker and podman. The problem is that to provide a 'build reproducer', one needs to be picked. And then if a user uses the other, they have to know that they can specify the other and how, or, we need to produce several reproducers, or, we need the reproducer to be a script which then starts to defeat the purpose.

What if `--runtime auto` did a `if $(podman); elif $(docker); else exit` to set --runtime dynamically at uh, runtime? So that our reproducers were portable between both supported container runtimes.

I looked but couldn't find any standard way to determine a user's preferred docker runtime.

*Migrated from GitLab: https://gitlab.com/Linaro/tuxmake/-/issues/92*

Start at the --runtime entry point and trace how the selected runtime is used when build reproducers are generated. Compare the existing docker and podman paths and define how auto should choose between them; done means reproducers work with either supported runtime without requiring a manual runtime selection.

Written by the indexing model from the issue text.