Expose HasRegistry bool to builder plugins, Docker doesn't pull if it doesn't have to

[mitchellh]

The main motivation for this PR is to enable a workflow where Waypoint is using an externally built image (such as from CI) without requiring docker or any privileges/namespaces even in a Dockerless environment (i.e. with img).

Since img still requires user namespaces and user namespaces aren't enabled by default on most hosted K8S platforms, this enables a working Waypoint experience if the above two conditions are met, guaranteed.

To make this work, I've exposed a named variable HasRegistry that builders can opt-in for (via the typical argmapper DI stuff, see the Docker implementation in this PR) to determine if there is a registry configured.

For Docker, if the CEB injection is disabled AND there is no registry, we don't pull at all.

[evanphx]

While I do like love pushing the envelope, probably the arg should be a named structure.

[mitchellh]

So turns out this doesn't work at all with real plugins cause we don't support named primitive types across the plugin boundary. Working on that now in waypoint-plugin-sdk.