bake: initial set of composable bake attributes

This allows using either the csv syntax or object syntax to specify
certain attributes.

This applies to the following fields:
- output
- cache-from
- cache-to
- secret
- ssh

There are still some remaining fields to translate. Specifically
ulimits, annotations, and attest.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

controller/pb/export.go

@@ -45,6 +45,7 @@ func CreateExports(entries []*ExportEntry) ([]client.ExportEntry, error) {
 			supportDir = !tar
 		case "registry":
 			out.Type = client.ExporterImage
+			out.Attrs["push"] = "true"
 		}
 
 		if supportDir {