Makefile: prefer podman for container engine

[loshz]

Ref: #479

Usage:

# If podman is found on the system, then it is preferred. Otherwise fall back to using docker.
# Passing the variable at runtime will always take precedence:
$ command -v podman
/bin/podman
$ CONTAINER_ENGINE=docker make container-all
docker run --rm --user "1000:985" \
...
$ make container-all
podman run --rm  \
...
# Override container run args
$ CONTAINER_RUN_ARGS='--user "1000:985"' make container-all
podman run --rm --user "1000:985" \
...
# Override both
$ CONTAINER_ENGINE=docker CONTAINER_RUN_ARGS='--name=cilium-ebpf' make container-all
docker run --rm --name=cilium-ebpf \
...