func (b *Builder) processImageFrom(img builder.Image) error {
	b.image = img.ID()

	if img.Config != nil {
		b.runConfig = img.Config()
	}

	// The default path will be blank on Windows (set by HCS)
	if len(b.runConfig.Env) == 0 && system.DefaultPathEnv != "" {
		b.runConfig.Env = append(b.runConfig.Env, "PATH="+system.DefaultPathEnv)
	}

	// Process ONBUILD triggers if they exist
	if nTriggers := len(b.runConfig.OnBuild); nTriggers != 0 {
		word := "trigger"
		if nTriggers > 1 {
			word = "triggers"
		}
		fmt.Fprintf(b.Stderr, "# Executing %d build %s...\n", nTriggers, word)
	}

	// Copy the ONBUILD triggers, and remove them from the config, since the config will be committed.
	onBuildTriggers := b.runConfig.OnBuild
	b.runConfig.OnBuild = []string{}

	// parse the ONBUILD triggers by invoking the parser
	for _, step := range onBuildTriggers {
		ast, err := parser.Parse(strings.NewReader(step))
		if err != nil {
			return err
		}

		for i, n := range ast.Children {
			switch strings.ToUpper(n.Value) {
			case "ONBUILD":
				return fmt.Errorf("Chaining ONBUILD via `ONBUILD ONBUILD` isn't allowed")
			case "MAINTAINER", "FROM":
				return fmt.Errorf("%s isn't allowed as an ONBUILD trigger", n.Value)
			}

			if err := b.dispatch(i, n); err != nil {
				return err
			}
		}
	}

	return nil
}

func (b *Builder) processImageFrom(img builder.Image) error {
	if img != nil {
		b.image = img.ImageID()

		if img.RunConfig() != nil {
			b.runConfig = img.RunConfig()
		}
	}

	// Check to see if we have a default PATH, note that windows won't
	// have one as its set by HCS
	if system.DefaultPathEnv != "" {
		// Convert the slice of strings that represent the current list
		// of env vars into a map so we can see if PATH is already set.
		// If its not set then go ahead and give it our default value
		configEnv := opts.ConvertKVStringsToMap(b.runConfig.Env)
		if _, ok := configEnv["PATH"]; !ok {
			b.runConfig.Env = append(b.runConfig.Env,
				"PATH="+system.DefaultPathEnv)
		}
	}

	if img == nil {
		// Typically this means they used "FROM scratch"
		return nil
	}

	// Process ONBUILD triggers if they exist
	if nTriggers := len(b.runConfig.OnBuild); nTriggers != 0 {
		word := "trigger"
		if nTriggers > 1 {
			word = "triggers"
		}
		fmt.Fprintf(b.Stderr, "# Executing %d build %s...\n", nTriggers, word)
	}

	// Copy the ONBUILD triggers, and remove them from the config, since the config will be committed.
	onBuildTriggers := b.runConfig.OnBuild
	b.runConfig.OnBuild = []string{}

	// parse the ONBUILD triggers by invoking the parser
	for _, step := range onBuildTriggers {
		ast, err := parser.Parse(strings.NewReader(step))
		if err != nil {
			return err
		}

		for i, n := range ast.Children {
			switch strings.ToUpper(n.Value) {
			case "ONBUILD":
				return fmt.Errorf("Chaining ONBUILD via `ONBUILD ONBUILD` isn't allowed")
			case "MAINTAINER", "FROM":
				return fmt.Errorf("%s isn't allowed as an ONBUILD trigger", n.Value)
			}

			if err := b.dispatch(i, n); err != nil {
				return err
			}
		}
	}

	return nil
}