Properly detect whether podman or docker should be used.

[GloriousEggroll]

Fedora ships podman with a podman-docker package that uses a bash script for /usr/bin/docker which runs exec /usr/bin/podman @, This does not work correctly with the current check mechanism in proton's configure script, resulting in invalid permission settings:

!! File owner's UID doesn't map to 0 or 1000 in the container.
!! Don't know how to map permissions. Please check your docker setup.

By checking if the /usr/bin/docker file contains contains 'bin/podman' we can easily determine whether or not to actually use podman before any other checks are done.

[ivyl]

Inspecting files like this feels fairly brittle. I think we can flip the order of the checks instead and assume that if a podman is there and is working that's what the person wants to use over docker.