fix: update CI docker-build to find Dockerfiles in correct paths

The docker-build job checked for root-level Dockerfile, but project
Dockerfiles live in docker/Dockerfile.backend, frontend/Dockerfile,
and android/Dockerfile. CI always skipped. Now builds all three.

Closes #45

.gitea/workflows/ci.yml

@@ -92,12 +92,23 @@ jobs:
           git clone --depth 1 $GITEA_URL/$GITHUB_REPOSITORY $GITHUB_WORKSPACE
           git -C $GITHUB_WORKSPACE checkout $GITHUB_SHA 2>/dev/null || true
 
-      - name: Build Docker image
+      - name: Build Docker images
         if: always()
         run: |
-          if [[ -f Dockerfile ]]; then
-            docker build -t $GITHUB_REPOSITORY:test .
-          else
+          built=false
+          if [[ -f docker/Dockerfile.backend ]]; then
+            docker build -t $GITHUB_REPOSITORY-backend:test -f docker/Dockerfile.backend .
+            built=true
+          fi
+          if [[ -f frontend/Dockerfile ]]; then
+            docker build -t $GITHUB_REPOSITORY-frontend:test -f frontend/Dockerfile .
+            built=true
+          fi
+          if [[ -f android/Dockerfile ]]; then
+            docker build -t $GITHUB_REPOSITORY-android:test -f android/Dockerfile android/
+            built=true
+          fi
+          if [[ "$built" != "true" ]]; then
             echo "No Dockerfile found, skipping docker build"
           fi