fix: use default builder with buildx (#3199)

Co-authored-by: CrazyMax <crazy-max@users.noreply.github.com>

internal/pipe/docker/api_docker.go

@@ -61,7 +61,7 @@ func (i dockerImager) Build(ctx *context.Context, root string, images, flags []s
 func (i dockerImager) buildCommand(images, flags []string) []string {
 	base := []string{"build", "."}
 	if i.buildx {
-		base = []string{"buildx", "build", ".", "--load"}
+		base = []string{"buildx", "--builder", "default", "build", ".", "--load"}
 	}
 	for _, image := range images {
 		base = append(base, "-t", image)

internal/pipe/docker/docker_test.go

@@ -1122,7 +1122,7 @@ func TestBuildCommand(t *testing.T) {
 			name:   "buildx",
 			buildx: true,
 			flags:  []string{"--label=foo", "--build-arg=bar=baz"},
-			expect: []string{"buildx", "build", ".", "--load", "-t", images[0], "-t", images[1], "--label=foo", "--build-arg=bar=baz"},
+			expect: []string{"buildx", "--builder", "default", "build", ".", "--load", "-t", images[0], "-t", images[1], "--label=foo", "--build-arg=bar=baz"},
 		},
 	}
 	for _, tt := range tests {

www/docs/customization/docker.md

@@ -240,6 +240,27 @@ docker build -t myuser/myimage . \
 !!! tip
     Learn more about the [name template engine](/customization/templates/).
 
+## Use a specific builder with Docker buildx
+
+If `buildx` is enabled, the `default` context builder will be used when building
+the image. This builder is always available and backed by BuildKit in the
+Docker engine. If you want to use a different builder, you can specify it using
+the `build_flag_templates` field:
+
+```yaml
+# .goreleaser.yaml
+dockers:
+  -
+    image_templates:
+    - "myuser/myimage"
+    use: buildx
+    build_flag_templates:
+    - "--builder=mybuilder"
+```
+
+!!! tip
+    Learn more about the [buildx builder instances](https://docs.docker.com/buildx/working-with-buildx/#work-with-builder-instances).
+
 ## Podman
 
 !!! success "GoReleaser Pro"