command(bake): Specify local and remote bake files

This adds the ability to source additional local build definition files when
sourcing Bake files via a remote url.
Prefixing a file with 'local://' will source the bake file on the local
machine, instead of the remote location.
Local files will be read/have precedence before remote files.

Usage:
```
docker buildx bake https://github.com/example/upstream.git --file local://docker-bake.override.hcl --print
```
This will source a default file from the example/upstream repository,
and also source a build definition from the local machine.

Signed-off-by: Cameron Adams <pnzreba@gmail.com>

commands/bake.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"os"
+	"strings"
 
 	"github.com/containerd/containerd/platforms"
 	"github.com/docker/buildx/bake"
@@ -134,7 +135,18 @@ func runBake(dockerCli command.Cli, targets []string, in bakeOptions, cFlags com
 	}()
 
 	if url != "" {
-		files, inp, err = bake.ReadRemoteFiles(ctx, nodes, url, in.files, printer)
+		if containsLocalFiles(in.files) {
+			var remoteFiles []bake.File
+			remoteFiles, inp, err = bake.ReadRemoteFiles(ctx, nodes, url, getRemoteFiles(in.files), printer)
+			if err != nil {
+				return err
+			}
+			files, err = bake.ReadLocalFiles(getLocalFiles(in.files))
+			files = append(files, remoteFiles...)
+
+		} else {
+			files, inp, err = bake.ReadRemoteFiles(ctx, nodes, url, in.files, printer)
+		}
 	} else {
 		files, err = bake.ReadLocalFiles(in.files)
 	}
@@ -247,3 +259,32 @@ func bakeCmd(dockerCli command.Cli, rootOpts *rootOptions) *cobra.Command {
 
 	return cmd
 }
+
+func containsLocalFiles(s []string) bool {
+	for _, v := range s {
+		if strings.Contains(v, "local://") {
+			return true
+		}
+	}
+	return false
+}
+
+func getLocalFiles(s []string) []string {
+	var files []string
+	for _, v := range s {
+		if strings.Contains(v, "local://") {
+			files = append(files, strings.TrimPrefix(v, "local://"))
+		}
+	}
+	return files
+}
+
+func getRemoteFiles(s []string) []string {
+	var files []string
+	for _, v := range s {
+		if !strings.Contains(v, "local://") {
+			files = append(files, v)
+		}
+	}
+	return files
+}