Fix regression in docker-tag post-processor (#8593)

GH-8392 introduced the feature that multiple tags can be specified. This
(presumably inadvertently) broke the option to tag an image without the
actual tag specified -- Docker handles this by assuming `latest` as the
tag.

In addition, use-cases exist for the `repository` field containing the
tag already, which was also broken.

CHANGELOG.md

@@ -10,6 +10,7 @@
 * builder/virtualbox-vm: use config as a non pointer to avoid a panic [GH-8576]
 * core: Fix crash when build.sources is set to an invalid name [GH-8569]
 * core: Fix loading of external plugins. GH-8543]
+* post-processor/docker-tag: Fix regression if no tags were specified. [GH-8593]
 * post-processor/vagrant: correctly handle the diskSize property as a qemu size
     string [GH-8567]
 * provisioner/ansible: Fix password sanitization to account for empty string

post-processor/docker-tag/post-processor.go

@@ -68,6 +68,7 @@ func (p *PostProcessor) PostProcess(ctx context.Context, ui packer.Ui, artifact
 
 	importRepo := p.config.Repository
 	var lastTaggedRepo = importRepo
+	if len(p.config.Tag) > 0 {
 		for _, tag := range p.config.Tag {
 			local := importRepo + ":" + tag
 			ui.Message("Tagging image: " + artifact.Id())
@@ -80,6 +81,14 @@ func (p *PostProcessor) PostProcess(ctx context.Context, ui packer.Ui, artifact
 
 			lastTaggedRepo = local
 		}
+	} else {
+		ui.Message("Tagging image: " + artifact.Id())
+		ui.Message("Repository: " + importRepo)
+		err := driver.TagImage(artifact.Id(), importRepo, p.config.Force)
+		if err != nil {
+			return nil, false, true, err
+		}
+	}
 
 	// Build the artifact
 	artifact = &docker.ImportArtifact{

post-processor/docker-tag/post-processor_test.go

@@ -127,3 +127,42 @@ func TestPostProcessor_PostProcess_Force(t *testing.T) {
 		t.Fatal("bad force")
 	}
 }
+
+func TestPostProcessor_PostProcess_NoTag(t *testing.T) {
+	driver := &docker.MockDriver{}
+	p := &PostProcessor{Driver: driver}
+	c := testConfig()
+	delete(c, "tag")
+	if err := p.Configure(c); err != nil {
+		t.Fatalf("err %s", err)
+	}
+
+	artifact := &packer.MockArtifact{BuilderIdValue: dockerimport.BuilderId, IdValue: "1234567890abcdef"}
+
+	result, keep, forceOverride, err := p.PostProcess(context.Background(), testUi(), artifact)
+	if _, ok := result.(packer.Artifact); !ok {
+		t.Fatal("should be instance of Artifact")
+	}
+	if !keep {
+		t.Fatal("should keep")
+	}
+	if !forceOverride {
+		t.Fatal("Should force keep no matter what user sets.")
+	}
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	if driver.TagImageCalled != 1 {
+		t.Fatal("should call TagImage")
+	}
+	if driver.TagImageImageId != "1234567890abcdef" {
+		t.Fatal("bad image id")
+	}
+	if driver.TagImageRepo[0] != "foo" {
+		t.Fatal("bad repo")
+	}
+	if driver.TagImageForce {
+		t.Fatal("bad force")
+	}
+}