Add ability to add multiple tags with docker build

Signed-off-by: Shijiang Wei <mountkin@gmail.com>

opts/opts.go

@@ -85,7 +85,6 @@ func (opts *ListOpts) Delete(key string) {
 
 // GetMap returns the content of values in a map in order to avoid
 // duplicates.
-// FIXME: can we remove this?
 func (opts *ListOpts) GetMap() map[string]struct{} {
 	ret := make(map[string]struct{})
 	for _, k := range *opts.values {
@@ -95,7 +94,6 @@ func (opts *ListOpts) GetMap() map[string]struct{} {
 }
 
 // GetAll returns the values of slice.
-// FIXME: Can we remove this?
 func (opts *ListOpts) GetAll() []string {
 	return (*opts.values)
 }