fix(manager/docker-compose): Allow strings or objects for service build (#27594)

Signed-off-by: Kyle Harding <kyle@balena.io>

lib/modules/manager/docker-compose/__fixtures__/docker-compose.1.yml

@@ -81,3 +81,6 @@ debugapp:
   build:
     context: .
     dockerfile: Dockerfile.local
+
+build-context:
+    build: ./debugapp

lib/modules/manager/docker-compose/schema.ts

@@ -3,10 +3,13 @@ import { z } from 'zod';
 const DockerComposeService = z.object({
   image: z.string().optional(),
   build: z
-    .object({
+    .union([
+      z.string(),
+      z.object({
         context: z.string().optional(),
         dockerfile: z.string().optional(),
-    })
+      }),
+    ])
     .optional(),
 });