system prune: use register function for prune functions

Introduce a "prune" package in which we maintain a list of prune
functions that are registered. Known prune "content-types" are
included in a pre-defined order, after which additional content
can be registered.

Using this approach no longer requires the "RunPrune" functions
to be exported, and allows additional content-types to be
introduced without having to import those packages into the
system package, so keeping things more decoupled.

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

cli/command/builder/prune.go

@@ -9,6 +9,7 @@ import (
 	"github.com/docker/cli/cli"
 	"github.com/docker/cli/cli/command"
 	"github.com/docker/cli/cli/command/completion"
+	"github.com/docker/cli/cli/command/system/pruner"
 	"github.com/docker/cli/internal/prompt"
 	"github.com/docker/cli/opts"
 	"github.com/docker/go-units"
@@ -16,6 +17,13 @@ import (
 	"github.com/spf13/cobra"
 )
 
+func init() {
+	// Register the prune command to run as part of "docker system prune"
+	if err := pruner.Register(pruner.TypeBuildCache, pruneFn); err != nil {
+		panic(err)
+	}
+}
+
 type pruneOptions struct {
 	force       bool
 	all         bool
@@ -105,6 +113,18 @@ type cancelledErr struct{ error }
 func (cancelledErr) Cancelled() {}
 
 // CachePrune executes a prune command for build cache
+//
+// Deprecated: this function was only used internally and will be removed in the next release.
 func CachePrune(ctx context.Context, dockerCli command.Cli, all bool, filter opts.FilterOpt) (uint64, string, error) {
 	return runPrune(ctx, dockerCli, pruneOptions{force: true, all: all, filter: filter})
 }
+
+// pruneFn prunes the build cache for use in "docker system prune" and
+// returns the amount of space reclaimed and a detailed output string.
+func pruneFn(ctx context.Context, dockerCLI command.Cli, options pruner.PruneOptions) (uint64, string, error) {
+	return runPrune(ctx, dockerCLI, pruneOptions{
+		force:  true,
+		all:    options.All,
+		filter: options.Filter,
+	})
+}