GitHub Actions cleanups (#24620)

- Remove actions name where command is descriptive enough
- Use kebab-case instead of snake-case for step names
- Use shorter job names because to make PR checks more readable
- Remove duplicate `checks-backend`

---------

Co-authored-by: Yarden Shoham <git@yardenshoham.com>

.github/workflows/pull-docker-dryrun.yml

@@ -0,0 +1,17 @@
+name: docker-dryrun
+
+on: [pull_request]
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
+jobs:
+  docker-dryrun:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: docker/setup-buildx-action@v2
+      - uses: docker/build-push-action@v4
+        with:
+          push: false
+          tags: gitea/gitea:linux-amd64