feat: Add support for docker build-flags

I added `BuildFlags` field to Docker image config. Docker pipe is
updated to append build flags to the `docker build` command.
Build flags are not validated, leaving it Docker to error
when given an unknown flag.

See #813

internal/pipe/docker/docker.go

@@ -128,7 +128,7 @@ func process(ctx *context.Context, docker config.Docker, artifact artifact.Artif
 	if err := os.Link(artifact.Path, filepath.Join(tmp, filepath.Base(artifact.Path))); err != nil {
 		return errors.Wrap(err, "failed to link binary")
 	}
-	if err := dockerBuild(ctx, tmp, images[0]); err != nil {
+	if err := dockerBuild(ctx, tmp, images[0], docker.BuildFlags); err != nil {
 		return err
 	}
 	for _, img := range images[1:] {
@@ -181,10 +181,10 @@ func publish(ctx *context.Context, docker config.Docker, images []string) error
 	return nil
 }
 
-func dockerBuild(ctx *context.Context, root, image string) error {
+func dockerBuild(ctx *context.Context, root, image string, flags []string) error {
 	log.WithField("image", image).Info("building docker image")
 	/* #nosec */
-	var cmd = exec.CommandContext(ctx, "docker", "build", "-t", image, ".")
+	var cmd = exec.CommandContext(ctx, "docker", buildCommand(image, flags)...)
 	cmd.Dir = root
 	log.WithField("cmd", cmd.Args).WithField("cwd", cmd.Dir).Debug("running")
 	out, err := cmd.CombinedOutput()
@@ -195,6 +195,12 @@ func dockerBuild(ctx *context.Context, root, image string) error {
 	return nil
 }
 
+func buildCommand(image string, flags []string) []string {
+	base := []string{"build", "-t", image, "."}
+	base = append(base, flags...)
+	return base
+}
+
 func dockerTag(ctx *context.Context, image, tag string) error {
 	log.WithField("image", image).WithField("tag", tag).Info("tagging docker image")
 	/* #nosec */

internal/pipe/docker/docker_test.go

@@ -235,6 +235,47 @@ func TestRunPipe(t *testing.T) {
 			},
 			assertError: shouldNotErr,
 		},
+		"valid build args": {
+			publish: false,
+			dockers: []config.Docker{
+				{
+					Image:      registry + "goreleaser/test_build_args",
+					Goos:       "linux",
+					Goarch:     "amd64",
+					Dockerfile: "testdata/Dockerfile",
+					Binary:     "mybin",
+					TagTemplates: []string{
+						"latest",
+					},
+					BuildFlags: []string{
+						"--label=foo=bar",
+					},
+				},
+			},
+			expect: []string{
+				registry + "goreleaser/test_build_args:latest",
+			},
+			assertError: shouldNotErr,
+		},
+		"bad build args": {
+			publish: false,
+			dockers: []config.Docker{
+				{
+					Image:      registry + "goreleaser/test_build_args",
+					Goos:       "linux",
+					Goarch:     "amd64",
+					Dockerfile: "testdata/Dockerfile",
+					Binary:     "mybin",
+					TagTemplates: []string{
+						"latest",
+					},
+					BuildFlags: []string{
+						"--bad-flag",
+					},
+				},
+			},
+			assertError: shouldErr("unknown flag: --bad-flag"),
+		},
 		"bad_dockerfile": {
 			publish: true,
 			dockers: []config.Docker{
@@ -415,6 +456,37 @@ func TestRunPipe(t *testing.T) {
 	}
 }
 
+func TestBuildCommand(t *testing.T) {
+	image := "goreleaser/test_build_flag"
+	tests := []struct {
+		name   string
+		flags  []string
+		expect []string
+	}{
+		{
+			name:   "no flags",
+			flags:  []string{},
+			expect: []string{"build", "-t", image, "."},
+		},
+		{
+			name:   "single flag",
+			flags:  []string{"--label=foo"},
+			expect: []string{"build", "-t", image, ".", "--label=foo"},
+		},
+		{
+			name:   "multiple flags",
+			flags:  []string{"--label=foo", "--build-arg=bar=baz"},
+			expect: []string{"build", "-t", image, ".", "--label=foo", "--build-arg=bar=baz"},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			command := buildCommand(image, tt.flags)
+			assert.Equal(t, tt.expect, command)
+		})
+	}
+}
+
 func TestDescription(t *testing.T) {
 	assert.NotEmpty(t, Pipe{}.String())
 }
@@ -508,6 +580,7 @@ func TestDefaultSet(t *testing.T) {
 	assert.Equal(t, []string{"{{ .Version }}"}, docker.TagTemplates)
 	assert.Equal(t, "Dockerfile.foo", docker.Dockerfile)
 }
+
 func TestLinkFile(t *testing.T) {
 	const srcFile = "/tmp/test"
 	const dstFile = "/tmp/linked"

pkg/config/config.go

@@ -247,6 +247,7 @@ type Docker struct {
 	SkipPush     bool     `yaml:"skip_push,omitempty"`
 	TagTemplates []string `yaml:"tag_templates,omitempty"`
 	Files        []string `yaml:"extra_files,omitempty"`
+	BuildFlags   []string `yaml:"build_flags,omitempty"`
 }
 
 // Filters config