fix: use woodpeckerci/plugin-docker-buildx for privileged builds

plugins/docker does not get privileged: true in K8s pods despite being
in WOODPECKER_PLUGINS_PRIVILEGED. woodpeckerci/plugin-docker-buildx
correctly receives privileged mode. Previous build failure with buildx
was a transient network timeout reaching registry-1.docker.io.

.woodpecker.yml

@@ -30,11 +30,9 @@ steps:
         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:
@@ -45,17 +43,13 @@ steps:
       build_args:
         - EXTRAS=api,news,sentiment,trading,backtester
         - SERVICE_MODULE=api_gateway
-      cache_from:
-        - 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:
@@ -65,8 +59,6 @@ steps:
       context: .
       build_args:
         - NGINX_CONF=docker/nginx-k8s.conf
-      cache_from:
-        - viktorbarzin/trading-bot-dashboard:latest
       tags:
         - "build-${CI_PIPELINE_NUMBER}"