fix: switch to buildx plugin and add pip caching for CI

- Use woodpeckerci/plugin-docker-buildx instead of plugins/docker to fix
  Docker daemon connection failures (privileged mode not applied)
- Add pip dependency caching between builds via /woodpecker/pip-cache
- Fix slack plugin image (plugins/slack, not woodpeckerci/plugin-slack)
- Use proper buildx cache_from syntax (type=registry,ref=...)

.woodpecker.yml

@@ -13,18 +13,26 @@ steps:
   - name: test
     image: python:3.12-slim
     commands:
+      - |
+        if [ -d /woodpecker/pip-cache ]; then
+          echo "Restoring pip cache..."
+          cp -r /woodpecker/pip-cache .pip-cache
+        fi
       - python -m venv .venv
-      - .venv/bin/pip install --quiet --upgrade pip
       - >-
-        .venv/bin/pip install --quiet ".[api,news,sentiment,trading,backtester,dev]"
+        .venv/bin/pip install --quiet --upgrade pip
+      - >-
+        .venv/bin/pip install --quiet --cache-dir .pip-cache
+        ".[api,news,sentiment,trading,backtester,dev]"
       - .venv/bin/pytest tests/ -v --tb=short -m "not integration"
+      - |
+        echo "Saving pip cache..."
+        cp -r .pip-cache /woodpecker/pip-cache 2>/dev/null || true
 
   - name: build-service-image
-    image: plugins/docker
+    image: woodpeckerci/plugin-docker-buildx
     depends_on:
       - test
-    environment:
-      DOCKER_BUILDKIT: 1
     settings:
       username: viktorbarzin
       password:
@@ -36,16 +44,14 @@ steps:
         - EXTRAS=api,news,sentiment,trading,backtester
         - SERVICE_MODULE=api_gateway
       cache_from:
-        - viktorbarzin/trading-bot-service:latest
+        - type=registry,ref=viktorbarzin/trading-bot-service:latest
       tags:
         - "build-${CI_PIPELINE_NUMBER}"
 
   - name: build-dashboard-image
-    image: plugins/docker
+    image: woodpeckerci/plugin-docker-buildx
     depends_on:
       - test
-    environment:
-      DOCKER_BUILDKIT: 1
     settings:
       username: viktorbarzin
       password:
@@ -56,7 +62,7 @@ steps:
       build_args:
         - NGINX_CONF=docker/nginx-k8s.conf
       cache_from:
-        - viktorbarzin/trading-bot-dashboard:latest
+        - type=registry,ref=viktorbarzin/trading-bot-dashboard:latest
       tags:
         - "build-${CI_PIPELINE_NUMBER}"
 
@@ -214,7 +220,7 @@ steps:
         done
 
   - name: slack
-    image: woodpeckerci/plugin-slack
+    image: plugins/slack
     depends_on:
       - verify-deploy
     settings: