feat: build containers for iperf with github actions

[kashalls]

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• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies: master

• Issues fixed (if any): (See brief description)

• Brief description of code changes (suitable for use as a commit message):

Adds automation to expand on pre-existing Dockerfile. It builds and deploys the container to ghcr.io/esnet/iperf.

[kashalls]

My intention for this feature was to setup some kind of "official" implementation of an iperf container.

A Dockerfile as added to /contrib in #824 which functions as a minimal docker container. This hasn't been updated and does not build the binary. This change builds iperf and puts it into a container that defaults to server mode.

If you go to docker hub, there are almost 1,400 different containers that have been pushed once and never again. There is no telling what those containers actually run unless you look into it. What individual is going to look through 1,400 containers.

Publishing this container, automatically, will allow users who only want to run trusted containers an option instead of having to build, maintain and deploy forks.

[userdocs]

I do something similar here but I use Alpine Linux and target more architectures + windows x86_64 builds.

https://github.com/userdocs/iperf3-static

docker run -it ghcr.io/userdocs/iperf3-static:latest iperf3 --version