cloudapi: support container embedding

Add support for embedding container images via the cloud API. For
this the container resolve job was plumbed into the cloud api's
handler and the API specification updated with a new `containers`
section that mimics the blueprint section with the same name.

internal/cloudapi/v2/handler.go

@@ -173,6 +173,19 @@ func (h *apiHandlers) PostCompose(ctx echo.Context) error {
 		}
 	}
 
+	if request.Customizations != nil && request.Customizations.Containers != nil {
+		for _, c := range *request.Customizations.Containers {
+			bc := blueprint.Container{
+				Source:    c.Source,
+				TLSVerify: c.TlsVerify,
+			}
+			if c.Name != nil {
+				bc.Name = *c.Name
+			}
+			bp.Containers = append(bp.Containers, bc)
+		}
+	}
+
 	if request.Customizations != nil && request.Customizations.Filesystem != nil {
 		var fsCustomizations []blueprint.FilesystemCustomization
 		for _, f := range *request.Customizations.Filesystem {