feat(build): add CS_N_PATH to override custom service build context

internal/compose/custom_services.go

@@ -52,13 +52,18 @@ func coreEnvVars(cfg *config.Config, svc config.CustomService) map[string]string
 
 // buildCustomService returns the service configuration for a user-defined
 // custom service (CS_1..CS_10). Each custom service is built from a Dockerfile
-// in ./services/{name}/ and exposes a single port with a /health endpoint.
+// in ./services/{name}/ by default, or from CS_N_PATH when set.
 func (g *Generator) buildCustomService(cs config.CustomService) ServiceConfig {
 	cfg := g.cfg
 
+	buildContext := cs.BuildPath
+	if buildContext == "" {
+		buildContext = fmt.Sprintf("./services/%s", cs.Name)
+	}
+
 	return ServiceConfig{
 		Build: &BuildConfig{
-			Context:    fmt.Sprintf("./services/%s", cs.Name),
+			Context:    buildContext,
 			Dockerfile: "Dockerfile",
 		},
 		ContainerName: fmt.Sprintf("%s_%s", cfg.ProjectName, cs.Name),

internal/config/custom_services.go

@@ -68,6 +68,20 @@ func parseCustomServices() ([]CustomService, error) {
 		cs.TablePrefix = os.Getenv(fmt.Sprintf("CS_%d_TABLE_PREFIX", i))
 		cs.ExtraEnv = os.Getenv(fmt.Sprintf("CS_%d_ENV", i))
 
+		// Optional build context path override. Rejects absolute paths and
+		// path traversal so a misconfigured env can't escape the project root.
+		if p := os.Getenv(fmt.Sprintf("CS_%d_PATH", i)); p != "" {
+			if strings.HasPrefix(p, "/") {
+				return nil, fmt.Errorf("CS_%d_PATH must be a relative path, got %q", i, p)
+			}
+			for _, seg := range strings.Split(p, "/") {
+				if seg == ".." {
+					return nil, fmt.Errorf("CS_%d_PATH must not contain '..', got %q", i, p)
+				}
+			}
+			cs.BuildPath = p
+		}
+
 		// Override port/route if explicitly set
 		if p := getEnvInt(fmt.Sprintf("CS_%d_PORT", i), 0); p != 0 {
 			cs.Port = p

internal/config/parse_services_test.go

@@ -82,6 +82,57 @@ func TestCustomServicesSanitization_PathTraversal(t *testing.T) {
 	}
 }
 
+// TestCustomServicesBuildPath_Valid verifies that CS_N_PATH sets BuildPath
+// when the value is a clean relative path.
+func TestCustomServicesBuildPath_Valid(t *testing.T) {
+	t.Setenv("CS_1", "myservice:go")
+	t.Setenv("CS_1_PATH", "./backend/services/myservice")
+	for i := 2; i <= 10; i++ {
+		t.Setenv("CS_"+itoa(i), "")
+	}
+
+	services, err := parseCustomServices()
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if len(services) == 0 {
+		t.Fatal("expected at least one custom service")
+	}
+	if got := services[0].BuildPath; got != "./backend/services/myservice" {
+		t.Errorf("BuildPath = %q, want %q", got, "./backend/services/myservice")
+	}
+}
+
+// TestCustomServicesBuildPath_AbsoluteRejected verifies that an absolute path
+// in CS_N_PATH is rejected.
+func TestCustomServicesBuildPath_AbsoluteRejected(t *testing.T) {
+	t.Setenv("CS_1", "myservice:go")
+	t.Setenv("CS_1_PATH", "/etc/secrets")
+	for i := 2; i <= 10; i++ {
+		t.Setenv("CS_"+itoa(i), "")
+	}
+
+	_, err := parseCustomServices()
+	if err == nil {
+		t.Fatal("expected error for absolute CS_N_PATH, got nil")
+	}
+}
+
+// TestCustomServicesBuildPath_TraversalRejected verifies that a path containing
+// '..' in CS_N_PATH is rejected.
+func TestCustomServicesBuildPath_TraversalRejected(t *testing.T) {
+	t.Setenv("CS_1", "myservice:go")
+	t.Setenv("CS_1_PATH", "../../outside")
+	for i := 2; i <= 10; i++ {
+		t.Setenv("CS_"+itoa(i), "")
+	}
+
+	_, err := parseCustomServices()
+	if err == nil {
+		t.Fatal("expected error for traversal CS_N_PATH, got nil")
+	}
+}
+
 // ---------------------------------------------------------------------------
 // T07 — Frontend apps sanitization
 // ---------------------------------------------------------------------------

internal/config/types.go

@@ -499,6 +499,7 @@ type CustomService struct {
 	CPU         string
 	TablePrefix string // CS_N_TABLE_PREFIX
 	ExtraEnv    string // CS_N_ENV (raw key=val pairs, comma-separated)
+	BuildPath   string // CS_N_PATH: overrides default ./services/{name} build context
 }
 
 // FrontendApp represents a frontend application (FRONTEND_APP_1..FRONTEND_APP_20).