base-images: Support podman

[gogolok]

I do not have docker running on my macos and FreeBSD system, but podman.

Sample run on my macos:

➜  base-images git:(support_podman) ✗ export CONTAINER_RUNTIME=podman
➜  base-images git:(support_podman) ✗ ./build.sh noble
Resolved "ubuntu" as an alias (/etc/containers/registries.conf.d/000-shortnames.conf)
Trying to pull docker.io/library/ubuntu:noble...

...


Successfully tagged localhost/cnbs/sample-base-run:noble
9845fd9b5b3a573e72d1a21b0e69a7cddcce668be55a7734b24185f9ba8e2425

BASE IMAGES BUILT!

Images:
    cnbs/sample-base:noble
    cnbs/sample-base-build:noble
    cnbs/sample-base-run:noble


➜  base-images git:(support_podman) ✗ podman images
REPOSITORY                        TAG         IMAGE ID      CREATED             SIZE
localhost/cnbs/sample-base-run    noble       9845fd9b5b3a  30 seconds ago      84.3 MB
localhost/cnbs/sample-base-build  noble       fb1bd63b452b  34 seconds ago      173 MB
localhost/cnbs/sample-base        noble       3f413700a6fc  About a minute ago  90.8 MB
<none>                            <none>      d90f11c075cd  2 weeks ago         103 MB
docker.io/library/ubuntu          noble       493218ed0f40  2 weeks ago         80.6 MB
docker.io/cnbs/sample-base        noble       ef15806a9951  2 months ago        84.3 MB

[AidanDelaney]

This is a great change and I'd like to see us show more independence from a single vendor runtime.