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.

.drone.yml

@@ -109,12 +109,25 @@ trigger:
     - push
 
 steps:
+  - name: Cache test image
+    image: plugins/docker
+    settings:
+      username: viktorbarzin
+      password:
+        from_secret: dockerhub-token
+      repo: viktorbarzin/realestatecrawler
+      dockerfile: Dockerfile
+      context: .
+      target: test
+      cache_from:
+        - viktorbarzin/realestatecrawler:test
+        - viktorbarzin/realestatecrawler:builder
+      tags:
+        - test
+
   - name: Run backend tests
-    image: python:3.13-slim
+    image: viktorbarzin/realestatecrawler:test
     commands:
-      - apt-get update && apt-get install -y --no-install-recommends gcc libmariadb-dev pkg-config python3-dev build-essential libgl1 libglib2.0-0 && rm -rf /var/lib/apt/lists/*
-      - pip install --no-cache-dir -r requirements.txt
-      - pip install --no-cache-dir pytest pytest-asyncio pytest-xdist httpx aioresponses fakeredis
       - pytest tests/ -x -q
 
   - name: Cache builder stage