Fix system prune warning missing filters from config-file

The warning, printed before runing docker system prune was missing any filter
that was set in the configuration file. In addition, the warning prefixes the
filters with `label=`, which is no longer accurate, now that the prune command
also supports "until" as a filter.

Before this change, only the filters set on the command-line were shown,
and any filter set in the configuration file was missing;

```
mkdir -p ./test-config
echo '{"pruneFilters": ["label!=never=remove-me", "label=remove=me"]}' > test-config/config.json
docker --config=./test-config system prune --filter until=24h --filter label=hello-world --filter label!=foo=bar --filter label=bar=baz

WARNING! This will remove:
        - all stopped containers
        - all networks not used by at least one container
        - all dangling images
        - all dangling build cache
        - Elements to be pruned will be filtered with:
        - label={"label":{"bar=baz":true,"hello-world":true},"label!":{"foo=bar":true},"until":{"24h":true}}
Are you sure you want to continue? [y/N]
```

With this patch applied, both options from the commandline and options set
in the configuration file are shown;

```
mkdir -p ./test-config
echo '{"pruneFilters": ["label!=never=remove-me", "label=remove=me"]}' > test-config/config.json
docker --config=./test-config system prune --filter until=24h --filter label=hello-world --filter label!=foo=bar --filter label=bar=baz

WARNING! This will remove:
        - all stopped containers
        - all networks not used by at least one container
        - all dangling images
        - all dangling build cache
        - Elements to be pruned will be filtered with:
        - filter={"label":{"bar=baz":true,"hello-world":true,"remove=me":true},"label!":{"foo=bar":true,"never=remove-me":true},"until":{"24h":true}}
```

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/system/prune.go

@@ -13,6 +13,7 @@ import (
 	"github.com/docker/cli/cli/command/network"
 	"github.com/docker/cli/cli/command/volume"
 	"github.com/docker/cli/opts"
+	"github.com/docker/docker/api/types/filters"
 	"github.com/docker/docker/api/types/versions"
 	units "github.com/docker/go-units"
 	"github.com/spf13/cobra"
@@ -63,7 +64,7 @@ func runPrune(dockerCli command.Cli, options pruneOptions) error {
 	if options.pruneVolumes && options.filter.Value().Contains("until") {
 		return fmt.Errorf(`ERROR: The "until" filter is not supported with "--volumes"`)
 	}
-	if !options.force && !command.PromptForConfirmation(dockerCli.In(), dockerCli.Out(), confirmationMessage(options)) {
+	if !options.force && !command.PromptForConfirmation(dockerCli.In(), dockerCli.Out(), confirmationMessage(dockerCli, options)) {
 		return nil
 	}
 	pruneFuncs := []func(dockerCli command.Cli, all bool, filter opts.FilterOpt) (uint64, string, error){
@@ -96,7 +97,7 @@ func runPrune(dockerCli command.Cli, options pruneOptions) error {
 }
 
 // confirmationMessage constructs a confirmation message that depends on the cli options.
-func confirmationMessage(options pruneOptions) string {
+func confirmationMessage(dockerCli command.Cli, options pruneOptions) string {
 	t := template.Must(template.New("confirmation message").Parse(confirmationTemplate))
 
 	warnings := []string{
@@ -118,9 +119,14 @@ func confirmationMessage(options pruneOptions) string {
 			warnings = append(warnings, "all dangling build cache")
 		}
 	}
-	if len(options.filter.String()) > 0 {
+	pruneFilters := command.PruneFilters(dockerCli, options.filter.Value())
+	if pruneFilters.Len() > 0 {
+		f, err := filters.ToJSON(pruneFilters)
+		if err != nil {
+			f = "invalid filters"
+		}
 		warnings = append(warnings, "Elements to be pruned will be filtered with:")
-		warnings = append(warnings, "label="+options.filter.String())
+		warnings = append(warnings, "filter="+f)
 	}
 
 	var buffer bytes.Buffer

cli/command/system/prune_test.go

@@ -3,6 +3,7 @@ package system
 import (
 	"testing"
 
+	"github.com/docker/cli/cli/config/configfile"
 	"github.com/docker/cli/internal/test"
 	"gotest.tools/assert"
 	is "gotest.tools/assert/cmp"
@@ -11,6 +12,7 @@ import (
 func TestPrunePromptPre131DoesNotIncludeBuildCache(t *testing.T) {
 	cli := test.NewFakeCli(&fakeClient{version: "1.30"})
 	cmd := newPruneCommand(cli)
+	cmd.SetArgs([]string{})
 	assert.NilError(t, cmd.Execute())
 	expected := `WARNING! This will remove:
         - all stopped containers
@@ -18,5 +20,23 @@ func TestPrunePromptPre131DoesNotIncludeBuildCache(t *testing.T) {
         - all dangling images
 Are you sure you want to continue? [y/N] `
 	assert.Check(t, is.Equal(expected, cli.OutBuffer().String()))
-
+}
+
+func TestPrunePromptFilters(t *testing.T) {
+	cli := test.NewFakeCli(&fakeClient{version: "1.30"})
+	cli.SetConfigFile(&configfile.ConfigFile{
+		PruneFilters: []string{"label!=never=remove-me", "label=remove=me"},
+	})
+	cmd := newPruneCommand(cli)
+	cmd.SetArgs([]string{"--filter", "until=24h", "--filter", "label=hello-world", "--filter", "label!=foo=bar", "--filter", "label=bar=baz"})
+
+	assert.NilError(t, cmd.Execute())
+	expected := `WARNING! This will remove:
+        - all stopped containers
+        - all networks not used by at least one container
+        - all dangling images
+        - Elements to be pruned will be filtered with:
+        - filter={"label":{"bar=baz":true,"hello-world":true,"remove=me":true},"label!":{"foo=bar":true,"never=remove-me":true},"until":{"24h":true}}
+Are you sure you want to continue? [y/N] `
+	assert.Check(t, is.Equal(expected, cli.OutBuffer().String()))
 }