Initial commit: add Modbus slave simulator

- TCP/RTU/TLS server support with pymodbus
- Configurable data store sizes and initial values
- CLI with argument parsing for port, slave-id, etc.
- GitHub Actions CI/CD for Docker build and push

.github/workflows/docker-publish.yml

@@ -0,0 +1,53 @@
+name: Build and Push to Docker Hub
+
+on:
+  push:
+    branches: [main]
+    tags:
+      - 'v*'
+  pull_request:
+    branches: [main]
+
+env:
+  REGISTRY: docker.io
+  IMAGE_NAME: ${{ secrets.DOCKERHUB_USERNAME }}/modbus-slave
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Login to Docker Hub
+        if: github.event_name != 'pull_request'
+        uses: docker/login-action@v3
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
+      - name: Extract metadata
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.IMAGE_NAME }}
+          tags: |
+            type=ref,event=branch
+            type=ref,event=pr
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=sha,prefix=
+
+      - name: Build and push
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          push: ${{ github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max