Apply relocation mapping to invocation image

Fixes cnabio#785

cmd/duffle/main.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"encoding/json"
 	"fmt"
 	"io/ioutil"
 	"os"
@@ -133,14 +134,39 @@ func (d *driverWithRelocationMapping) Run(op *driver.Operation) error {
 	// if there is a relocation mapping, ensure it is mounted
 	if d.relMapping != "" {
 		op.Files["/cnab/app/relocation-mapping.json"] = d.relMapping
+
+		var err error
+		op, err = d.relocateInvocationImage(op)
+		if err != nil {
+			return err
+		}
 	}
+
 	return d.driver.Run(op)
 }
 
 func (d *driverWithRelocationMapping) Handles(it string) bool {
 	return d.driver.Handles(it)
 }
 
+func (d *driverWithRelocationMapping) relocateInvocationImage(op *driver.Operation) (*driver.Operation, error) {
+	relMap := make(map[string]string)
+	err := json.Unmarshal([]byte(d.relMapping), &relMap)
+	if err != nil {
+		return nil, err
+	}
+
+	// TODO: copy op before mutating it
+
+	var ok bool
+	op.Image, ok = relMap[op.Image]
+	if !ok {
+		return nil, fmt.Errorf("invocation image %s not present in relocation mapping %v", op.Image, relMap)
+	}
+
+	return op, nil
+}
+
 func loadRelMapping(relMap string) (string, error) {
 	if relMap != "" {
 		data, err := ioutil.ReadFile(relMap)