Provide 'podman' as a build backend

[yuvipanda]

Proposed change

Currently, only docker is available to build images. We should:

1. Abstract the interface that builds a Dockerfile
2. Implement docker (via docker-py) as the default implementation
3. Allow podman to be the second implementation, so we can make sure our design isn't docker specific.

podman supports building Dockerfiles directly, so this should be a fairly minimal change.

Alternative options

We do (1) and (2) in core, but leave (3) to be a plugin.

Who would use this feature?

Anyone who doesn't want to use the docker daemon. There are good technical and social reasons for this.

How much effort will adding it take?

Abstracting the current docker calls behind an interface shouldn't be too much work. Podman is going to be interesting. It seems pretty production ready, but treats running on ubuntu as an extreme second class citizen - I haven't been able to get it to run yet. So that might be more challenging - making the case for keeping the podman build bits in an extension.

Who can do this work?

You'd need some familiarity with the docker python client and Dockerfile.

[betatim]

Are there other backends we should also take a look at (not implement though) to inform how we design the abstraction layer?

👍 on attempting to work towards repo2docker being able to use things that aren't docker!

[yuvipanda]

To begin with I think we should only support things that understand Dockerfiles. Off the top of my head, that is BuildKit, Podman and possibly buildah

[yuvipanda]

See also #560