update: use DOCKER_HOST as the default

internal/build/phase_config_provider.go

@@ -156,7 +156,7 @@ func WithDaemonAccess(dockerHost string) PhaseConfigProviderOperation {
 	return func(provider *PhaseConfigProvider) {
 		WithRoot()(provider)
 		if dockerHost == "inherit" {
-			dockerHost = os.Getenv("DOCKER_HOST")
+			dockerHost = strings.TrimSpace(os.Getenv("DOCKER_HOST"))
 		}
 		var bind string
 		if dockerHost == "" {

internal/build/phase_config_provider_test.go

@@ -171,6 +171,47 @@ func testPhaseConfigProvider(t *testing.T, when spec.G, it spec.S) {
 					h.AssertSliceContains(t, phaseConfigProvider.HostConfig().Binds, `\\.\pipe\docker_engine:\\.\pipe\docker_engine`)
 				})
 			})
+
+			when("DOCKER_HOST is set", func() {
+				it("should use the value", func() {
+					t.Setenv("DOCKER_HOST", "tcp://127.0.0.1:2375")
+
+					fakeBuilderImage := ifakes.NewImage("fake-builder", "", nil)
+					h.AssertNil(t, fakeBuilderImage.SetOS("osx"))
+
+					fakeBuilder, err := fakes.NewFakeBuilder(fakes.WithImage(fakeBuilderImage))
+					h.AssertNil(t, err)
+
+					lifecycle := newTestLifecycleExec(t, false, "some-temp-dir", fakes.WithBuilder(fakeBuilder))
+
+					phaseConfigProvider := build.NewPhaseConfigProvider(
+						"some-name",
+						lifecycle,
+						build.WithDaemonAccess("inherit"),
+					)
+					h.AssertSliceContains(t, phaseConfigProvider.ContainerConfig().Env, "DOCKER_HOST=tcp://127.0.0.1:2375")
+					h.AssertSliceNotContains(t, phaseConfigProvider.HostConfig().Binds, "/var/run/docker.sock:/var/run/docker.sock")
+				})
+			})
+
+			when("DOCKER_HOST is empty", func() {
+				it("falls back to /var/run/docker.sock", func() {
+					fakeBuilderImage := ifakes.NewImage("fake-builder", "", nil)
+					h.AssertNil(t, fakeBuilderImage.SetOS("osx"))
+
+					fakeBuilder, err := fakes.NewFakeBuilder(fakes.WithImage(fakeBuilderImage))
+					h.AssertNil(t, err)
+
+					lifecycle := newTestLifecycleExec(t, false, "some-temp-dir", fakes.WithBuilder(fakeBuilder))
+
+					phaseConfigProvider := build.NewPhaseConfigProvider(
+						"some-name",
+						lifecycle,
+						build.WithDaemonAccess("inherit"),
+					)
+					h.AssertSliceContains(t, phaseConfigProvider.HostConfig().Binds, "/var/run/docker.sock:/var/run/docker.sock")
+				})
+			})
 		})
 
 		when("called with WithEnv", func() {

internal/commands/build.go

@@ -242,10 +242,10 @@ func buildCommandFlags(cmd *cobra.Command, buildFlags *BuildFlags, cfg config.Co
 	cmd.Flags().StringArrayVar(&buildFlags.PreBuildpacks, "pre-buildpack", []string{}, "Buildpacks to prepend to the groups in the builder's order")
 	cmd.Flags().StringArrayVar(&buildFlags.PostBuildpacks, "post-buildpack", []string{}, "Buildpacks to append to the groups in the builder's order")
 	cmd.Flags().BoolVar(&buildFlags.Publish, "publish", false, "Publish to registry")
-	cmd.Flags().StringVar(&buildFlags.DockerHost, "docker-host", "",
+	cmd.Flags().StringVar(&buildFlags.DockerHost, "docker-host", "inherit",
 		`Address to docker daemon that will be exposed to the build container.
-If not set (or set to empty string) the standard socket location will be used.
-Special value 'inherit' may be used in which case DOCKER_HOST environment variable will be used.
+By default it's set to inherit and the DOCKER_HOST environment variable will be used.
+If set to empty string the standard socket location will be used.
 This option may set DOCKER_HOST environment variable for the build container if needed.
 `)
 	cmd.Flags().StringVar(&buildFlags.LifecycleImage, "lifecycle-image", cfg.LifecycleImage, `Custom lifecycle image to use for analysis, restore, and export when builder is untrusted.`)