Add build: fallback alongside image: for initial bootstrap

When both image: and build: are present, docker compose up --build
builds locally and tags with the image name. This allows the stack
to start before registry images exist, solving the bootstrap problem.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.yaml

@@ -20,6 +20,7 @@ services:
 
   backend:
     image: git.sentinelforest.xyz/rohskiddo/umbra-backend:main-latest
+    build: ./backend
     restart: unless-stopped
     env_file: .env
     depends_on:
@@ -42,6 +43,7 @@ services:
 
   frontend:
     image: git.sentinelforest.xyz/rohskiddo/umbra-frontend:main-latest
+    build: ./frontend
     restart: unless-stopped
     ports:
       - "80:8080"