feat: add Docker support and CI workflow

- Created a Dockerfile to build and deploy the application using a multi-stage build process.
- Added a GitHub Actions workflow to automatically build and publish the Docker image to GitHub Container Registry on pushes to the main branch.
- Updated go.mod and go.sum to reflect dependency upgrades and changes.
- Modified the RSS generator to extend the photo expiration time from 10 minutes to 12 hours.
- Updated rate limiter configuration to allow for environment variable customization.

.github/workflows/docker-publish.yml

@@ -0,0 +1,19 @@
+name: Build and publish a Docker image to ghcr.io
+on:
+  push:
+    branches:
+      - main
+
+jobs:
+  docker_publish:
+    runs-on: "ubuntu-latest"
+
+    steps:
+      - uses: actions/checkout@v2
+
+      # https://github.com/marketplace/actions/push-to-ghcr
+      - name: Build and publish a Docker image for ${{ github.repository }}
+        uses: macbre/push-to-ghcr@master
+        with:
+          image_name: ${{ github.repository }}
+          github_token: ${{ secrets.GITHUB_TOKEN }}