Use different format arg when using docker.

[ryanlovett]

Fixes #85.

[manics]

Thanks!

I'm going to do some investigation to see whether we can get away without the conditional formatting. E.g. podman image list --format '{{json .}}' outputs images as JSON-lines instead of an array. If that's the same as docker, and it works for all the other podman commands, it could be the default.

[ryanlovett]

Thanks @manics!