Cache test image to speed up CI backend test step

Add a 'test' stage to Dockerfile that extends runtime-base with the
venv and test dependencies (pytest, fakeredis, etc.) pre-installed.
Drone CI now builds and caches this image as :test, then uses it
directly for running tests — eliminating apt-get and pip install
on every build.

Dockerfile

@@ -28,7 +28,17 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     libmariadb3 \
     && rm -rf /var/lib/apt/lists/*
 
-# Stage 3: Final image — combine venv from builder + runtime base
+# Stage 3: Test image — runtime deps + venv + test dependencies
+FROM runtime-base AS test
+
+WORKDIR /app
+
+COPY --from=builder /app/.venv /app/.venv
+ENV PATH="/app/.venv/bin:$PATH"
+
+RUN pip install --no-cache-dir pytest pytest-asyncio pytest-xdist httpx aioresponses fakeredis
+
+# Stage 4: Final image — combine venv from builder + runtime base
 FROM runtime-base
 
 WORKDIR /app