docs: list available filters for du and prune commands

[crazy-max]

follow-up:

• Feature request: Pruning of layers of specific image #1065 (comment)
• Support --format=json for buildx du --verbose #3367 (comment)
• [Feature] Ability to inspect cache mounts #931

fixes #1922

Currently we have filters for the following keys: https://github.com/moby/buildkit/blob/3d43066f2e814d4d10383ba871256245edf08a25/cache/manager.go#L1641-L1659

Key type can be one of: https://github.com/moby/buildkit/blob/3d43066f2e814d4d10383ba871256245edf08a25/client/diskusage.go#L81-L88

We also have the until key that is used on client side to set KeepDuration for prune info request:

buildx/commands/prune.go

Lines 190 to 243 in 3f4bf82

	func toBuildkitPruneInfo(f filters.Args) (*client.PruneInfo, error) {
	var until time.Duration
	untilValues := f.Get("until") // canonical
	unusedForValues := f.Get("unused-for") // deprecated synonym for "until" filter
	if len(untilValues) > 0 && len(unusedForValues) > 0 {
	return nil, errors.Errorf("conflicting filters %q and %q", "until", "unused-for")
	}
	untilKey := "until"
	if len(unusedForValues) > 0 {
	untilKey = "unused-for"
	}
	untilValues = append(untilValues, unusedForValues...)
	switch len(untilValues) {
	case 0:
	// nothing to do
	case 1:
	var err error
	until, err = time.ParseDuration(untilValues[0])
	if err != nil {
	return nil, errors.Wrapf(err, "%q filter expects a duration (e.g., '24h')", untilKey)
	}
	default:
	return nil, errors.Errorf("filters expect only one value")
	}
	filters := make([]string, 0, f.Len())
	for _, filterKey := range f.Keys() {
	if filterKey == untilKey {
	continue
	}
	values := f.Get(filterKey)
	switch len(values) {
	case 0:
	filters = append(filters, filterKey)
	case 1:
	if filterKey == "id" {
	filters = append(filters, filterKey+"~="+values[0])
	} else if strings.HasSuffix(filterKey, "!") || strings.HasSuffix(filterKey, "~") {
	filters = append(filters, filterKey+"="+values[0])
	} else {
	filters = append(filters, filterKey+"=="+values[0])
	}
	default:
	return nil, errors.Errorf("filters expect only one value")
	}
	}
	return &client.PruneInfo{
	KeepDuration: until,
	Filter: []string{strings.Join(filters, ",")},
	}, nil
	}

We are using containerd module to parse filters: https://github.com/containerd/containerd/blob/7d8765d440e4991a7d005db19e7873651ce3551d/pkg/filters/parser.go#L26-L49

[tonistiigi]

@fenollp