builder/docker: fix issue where docker might get nil interp context

builder/docker/builder.go

@@ -26,7 +26,7 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 }
 
 func (b *Builder) Run(ui packer.Ui, hook packer.Hook, cache packer.Cache) (packer.Artifact, error) {
-	driver := &DockerDriver{Ctx: b.config.ctx, Ui: ui}
+	driver := &DockerDriver{Ctx: &b.config.ctx, Ui: ui}
 	if err := driver.Verify(); err != nil {
 		return nil, err
 	}

builder/docker/config.go

@@ -26,7 +26,7 @@ type Config struct {
 	LoginPassword string `mapstructure:"login_password"`
 	LoginServer   string `mapstructure:"login_server"`
 
-	ctx *interpolate.Context
+	ctx interpolate.Context
 }
 
 func NewConfig(raws ...interface{}) (*Config, []string, error) {