Dockerfile available for swiping

# Context

* Version of iperf3: current

* Hardware: anything that will run a Linux container

* Operating system (and distribution, if any): ditto

# Enhancement Request

* Current behavior: Upstream project does not offer an official container.

* Desired behavior: Has one. :)

* Implementation notes:

I believe I have perfected the `iperf3` container; [details here](https://tangentsoft.com/mikrotik/dir?name=iperf3&ci=trunk). It can't get any smaller without losing features, being a single static binary.

**EDIT:** Since publishing my initial Docker-baased version, I've tracked all releases of `iperf3` and converted to a Podman `Containerfile` with a "farm" based build system which publishes images to both Docker Hub and Quay. Being GitHub users, you might prefer GH build systems and GHCR. I understand you can make this automatic on making a release.

I hereby authorize this project to swipe my Containerfile and build rules [under your contribution guidelines](https://github.com/esnet/iperf/blob/97c2e40dfb575f68d0f72d80ed64e5cd7605c93b/.github/CONTRIBUTING.md). If you do not wish to publish images via GHCR, you may wish instead to [apply for official sponsored OSS status](https://www.docker.com/community/open-source/application/) with Docker Hub. That should rack up the download numbers for you, where for me, I've just barely cracked 10k pulls since posting this issue.