sanity.sh: Allow use of podman instead of docker

When running tests/sanity/sanity.sh locally, podman might be available
instead of Docker. Due to current configuration, only Docker is used by
sanity.sh.

This patch searches for the availability of docker, which is kept as
the default container engine to use, and use podman only if docker is
not found.

This change also allows the execution of the script from a directory
other than the repository root.