* add support to pull from private registry with --registry flags
* document registry username / password setting usage info
* remove use of deprecated Inspect call
* use extracted auth config
* assume dockerhub when we do not have a registry
* cleanup
* cleanup
* use registry account and secret
* add support for explicit docker-config flag
* update README, add docker config command
* update README, add docker config command
* fix shadowed err
Before this, I got build errors complaining about undefined logrus symbols from
the docker package, e.g.
```
../../vendor/github.com/docker/docker/errdefs/http_helpers.go:103:21: undefined: logrus.Fields
```
I suspect this happened because the path for the logrus package has a casing
error, but this did not surface on case-insensitive filesystems.
In addition, I ran `go mod vendor` and `go mod tidy`, which added some
additional dependencies to go.mod.