Add Docker CLI fallback if Podman is not found

[dagood]

Add a simple fallback to the logic that detects podman to also try to detect docker.

Related:

• Support use with Docker #164
  (A nice coincidence--I happened to be getting this change ready for PR anyway. 🙂)
• [BUG] Should work with docker #81

[pranavgaikwad]

I wonder if CONTAINER_RUNTIME would be a better name instead of PODMAN_BIN now that we know docker can be used as alternative? wdyt @dagood @eemcmullan @shawn-hurley

[shawn-hurley]

I am ok with PODMAN_BIN. I would worry about container_runtime, because I think that refers to something other than what we are using.

CONTAINER_TOOL or something might work though?

[pranavgaikwad]

You're right, that would be confusing...nvm, thanks!

[dagood]

Thanks for the reviews and merge!

I do think that a neutral name could make it easier to document (and understand)--the name PODMAN_BIN makes a pretty strong first impression. 😄 I was considering e.g. loadDefaultContainerizationCLI instead of loadDefaultPodmanBin, but stuck with PodmanBin to match the env var for now. I didn't want to propose changing the env var name at the same time as this PR because picking a good name might need some discussion, the change might need some iteration to find usages (and maybe break some people downstream), and auto-fallback makes the env var a little less prominent anyway.

That said, CONTAINER_TOOL sounds reasonable to me!

[shawn-hurley]

A follow up PR to rename and handle the existing PODMAN_BIN I think would be a good thing, if not can you file an issue and we can make the follow on ?

[dagood]

Thanks, filed #168.