ci: add working-tree image test and drop dead args step

- Build and smoke-test the local Dockerfile/entrypoint since the action
  now runs the published ghcr image instead of the working tree
- Exercise the empty INPUT_MESSAGE_THREAD_ID guard in the smoke test
- Remove the args step which silently sent the default message

.github/workflows/ci.yml

@@ -1,18 +1,34 @@
 name: telegram message
 on: [push]
 jobs:
+  # Tests the working-tree Dockerfile and entrypoint.sh; the build job below
+  # runs against the published image referenced by action.yml instead.
+  local-image:
+    name: Test local image build
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v7
+
+      - name: build image from working tree
+        run: docker build -t telegram-action:local .
+
+      - name: send message via local image
+        env:
+          TELEGRAM_TO: ${{ secrets.TELEGRAM_TO }}
+          TELEGRAM_TOKEN: ${{ secrets.TELEGRAM_TOKEN }}
+        run: |
+          docker run --rm \
+            -e INPUT_TO="$TELEGRAM_TO" \
+            -e INPUT_TOKEN="$TELEGRAM_TOKEN" \
+            -e INPUT_MESSAGE_THREAD_ID= \
+            -e INPUT_MESSAGE="ci: local image smoke test for ${{ github.sha }}" \
+            telegram-action:local
+
   build:
     name: Build
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v7
-      - name: send custom message with args
-        uses: ./
-        with:
-          to: ${{ secrets.TELEGRAM_TO }}
-          token: ${{ secrets.TELEGRAM_TOKEN }}
-          args: The ${{ github.event_name }} event triggered first step.
-
       - name: send message using with
         uses: ./
         with: