Add a dangling image prefix when tag name is not specified.

Fixes: #1398
Signed-off-by: Manu Gupta <manugupt1@gmail.com>

cmd/nerdctl/build.go

@@ -320,6 +320,8 @@ func generateBuildctlArgs(cmd *cobra.Command, buildkitHost string, platform, arg
 			}
 			tags[idx] = dockerreference.TagNameOnly(named).String()
 		}
+	} else if len(tags) == 0 {
+		output = output + ",dangling-name-prefix=none"
 	}
 
 	buildctlArgs = buildkitutil.BuildctlBaseArgs(buildkitHost)

cmd/nerdctl/build_test.go

@@ -17,6 +17,7 @@
 package main
 
 import (
+	"encoding/json"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -402,3 +403,33 @@ CMD ["echo", "dockerfile"]
 	base.Cmd("build", "-t", imageName, buildCtx).AssertOK()
 	base.Cmd("run", "--rm", imageName).AssertOutExactly("dockerfile\n")
 }
+
+func TestBuildNoTag(t *testing.T) {
+	testutil.RequiresBuild(t)
+	testutil.DockerIncompatible(t)
+	base := testutil.NewBase(t)
+	defer base.Cmd("builder", "prune").AssertOK()
+	dockerfile := fmt.Sprintf(`FROM %s
+CMD ["echo", "nerdctl-build-notag-string"]
+	`, testutil.CommonImage)
+	buildCtx, err := createBuildContext(dockerfile)
+	assert.NilError(t, err)
+	defer os.RemoveAll(buildCtx)
+
+	base.Cmd("build", buildCtx).AssertOK()
+	imgs := base.Cmd("images", "--format", "json").OutLines()
+	imgs = imgs[:len(imgs)-1]
+	var noneImage *imagePrintable
+	var foundNoneImage bool
+	for _, img := range imgs {
+		err := json.Unmarshal([]byte(img), &noneImage)
+		assert.NilError(t, err, img)
+		if noneImage.Tag == "<none>" {
+			foundNoneImage = true
+			break
+		}
+	}
+	assert.Assert(t, foundNoneImage)
+	defer base.Cmd("image", "rm", noneImage.ID).AssertOK()
+	base.Cmd("run", "--rm", noneImage.ID).AssertOutExactly("nerdctl-build-notag-string\n")
+}