Support podman in tasks/build.rake

[gregorynisbet-google]

Support podman in addition to docker in tasks/build.rake via the DOCKER environment variable.

Pull Request (PR) description

I did the simplest thing I could think of to make DOCKER=podman rake vox:build succeed.

There are a few differences between --format json for Docker and podman, but I still think
that that's the most robust way to get the commands to emit structured output.

I'm curious whether folks think this is worth doing at all, and whether an environment variable
is the best way of achieving this (and also whether shell substitution is better than doing the
conditional at the ruby level)

This Pull Request (PR) fixes the following issues

n/a

[Sharpie]

I opened #228 before I noticed this PR. The approach there is even simpler, but requires the use of the podman-docker shim package to re-direct the docker CLI to podman.

Building other OpenVox components, such as openvox-agent and openbolt, under Podman requires the use of podman-docker unless some enhancement is done to Vanagon.