feat: add SMTP proxy server (#177)

.github/workflows/smtp_proxy_server.yml

@@ -0,0 +1,40 @@
+name: SMTP Proxy Server Docker Image CI
+
+on:
+  push:
+    paths:
+      - "smtp_proxy_server/**"
+    tags:
+      - "*"
+    branches:
+      - main
+  workflow_dispatch:
+
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: smtp_proxy_server
+
+jobs:
+  build-and-push-image:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Log in to the Container registry
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Build and push Docker images
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          push: true
+          tags: |
+            ${{ env.REGISTRY }}/${{ github.repository }}/${{ env.IMAGE_NAME }}:${{ github.ref_name }}
+            ${{ env.REGISTRY }}/${{ github.repository }}/${{ env.IMAGE_NAME }}:latest