fix: set build_context to backend/ and frontend/ subdirectories

Dockerfiles use COPY requirements.txt . and COPY package*.json ./ which
expect the build context to be their own subdirectory, not the repo root.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/build-containers-on-demand.yml

@@ -21,13 +21,13 @@ jobs:
             image_yq_path: .frontend.image
             expected_repo: git.baumann.gr/adebaumann/shorefront-frontend
             dockerfile: frontend/Dockerfile
-            build_context: .
+            build_context: frontend
             description: shorefront frontend
           - container_name: shorefront-backend
             image_yq_path: .backend.image
             expected_repo: git.baumann.gr/adebaumann/shorefront-backend
             dockerfile: backend/Dockerfile
-            build_context: .
+            build_context: backend
             description: shorefront backend
 
     env: