prune: move image pruning before build cache pruning

This is cleaner because running system prune twice in a row
now results in a no-op the second time.

Signed-off-by: Tibor Vass <tibor@docker.com>

cli/command/image/prune.go

@@ -8,7 +8,9 @@ import (
 	"github.com/docker/cli/cli"
 	"github.com/docker/cli/cli/command"
 	"github.com/docker/cli/opts"
+	"github.com/docker/docker/api/types/filters"
 	units "github.com/docker/go-units"
+	"github.com/pkg/errors"
 	"github.com/spf13/cobra"
 )
 
@@ -55,8 +57,25 @@ Are you sure you want to continue?`
 Are you sure you want to continue?`
 )
 
+// cloneFilter is a temporary workaround that uses existing public APIs from the filters package to clone a filter.
+// TODO(tiborvass): remove this once filters.Args.Clone() is added.
+func cloneFilter(args filters.Args) (newArgs filters.Args, err error) {
+	if args.Len() == 0 {
+		return filters.NewArgs(), nil
+	}
+	b, err := args.MarshalJSON()
+	if err != nil {
+		return newArgs, err
+	}
+	err = newArgs.UnmarshalJSON(b)
+	return newArgs, err
+}
+
 func runPrune(dockerCli command.Cli, options pruneOptions) (spaceReclaimed uint64, output string, err error) {
-	pruneFilters := options.filter.Value()
+	pruneFilters, err := cloneFilter(options.filter.Value())
+	if err != nil {
+		return 0, "", errors.Wrap(err, "could not copy filter in image prune")
+	}
 	pruneFilters.Add("dangling", fmt.Sprintf("%v", !options.all))
 	pruneFilters = command.PruneFilters(dockerCli, pruneFilters)
 

cli/command/system/prune.go

@@ -73,11 +73,10 @@ func runPrune(dockerCli command.Cli, options pruneOptions) error {
 	if options.pruneVolumes {
 		pruneFuncs = append(pruneFuncs, volume.RunPrune)
 	}
+	pruneFuncs = append(pruneFuncs, image.RunPrune)
 	if options.pruneBuildCache {
 		pruneFuncs = append(pruneFuncs, builder.CachePrune)
 	}
-	// FIXME: modify image.RunPrune to not modify options.filter, otherwise this has to be last in the list.
-	pruneFuncs = append(pruneFuncs, image.RunPrune)
 
 	var spaceReclaimed uint64
 	for _, pruneFn := range pruneFuncs {