Improve Makefile usability with podman

[ti-mo]

Currently, in order to rebuild testdata using podman, make needs to be invoked as follows:

CONTAINER_ENGINE=podman CONTAINER_RUN_ARGS= make

This is a bit verbose and hard to remember. Perhaps the Makefile could run command -v podman and automatically use it when present on the machine?

Also, running rootless containers requires subuids and subgids to be allocated for the current user (see containers/podman#3421 (comment)).

~ cat /etc/subuid /etc/subgid
timo:100000:65536
timo:100000:65536

Since I didn't set it up before running podman for the first time, an extra podman system migrate was required to get containers to start properly.

Ideally, this would be documented as well.

[loshz]

I can take a stab at this if you like? Should be fairly straightforward.

My only thoughts are what if a user has podman installed, but still wants to use docker?