replace PR docker dry run in drone with Actions (#24475)

As before with past PRs to switch Drone pipelines to use (GitHub)
Actions.

---------

Co-authored-by: silverwind <me@silverwind.io>

.github/workflows/pull-docker_dryrun.yml

@@ -0,0 +1,19 @@
+name: Docker build dry run
+
+on: [pull_request]
+
+jobs:
+  docker_dryrun:
+    runs-on: ubuntu-latest
+    steps:
+      - name: checkout
+        uses: actions/checkout@v3
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v2
+      - name: Build and push
+        uses: docker/build-push-action@v4
+        with:
+          push: false
+          tags: gitea/gitea:linux-amd64
+          build-args: |
+            GOPROXY=https://goproxy.io