Add static homepage, Dockerfile (nginx), and CI build/push workflow

- index.html: clean static homepage (name + intro, links), light/dark mode
- Dockerfile: nginx:1.27-alpine serves the page on :80 with a healthcheck
- .gitea/workflows/build.yml: on push to main, build and push the image to
  git.ffaerber.duckdns.org/ffaerber/homepage (tags :latest and :sha)

.gitea/workflows/build.yml

@@ -0,0 +1,42 @@
+name: build-and-push
+run-name: "build-and-push · ${gitea.ref_name}"
+# Builds the homepage image and pushes it to this Gitea instance's container
+# registry as git.ffaerber.duckdns.org/ffaerber/homepage:latest on every push to main.
+on:
+  push:
+    branches: [main]
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  packages: write
+
+env:
+  IMAGE: git.ffaerber.duckdns.org/ffaerber/homepage
+
+jobs:
+  build-and-push:
+    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 container registry
+        uses: docker/login-action@v3
+        with:
+          registry: git.ffaerber.duckdns.org
+          username: ${{ gitea.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Build and push
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          push: true
+          tags: |
+            ${{ env.IMAGE }}:latest
+            ${{ env.IMAGE }}:${{ gitea.sha }}
+          provenance: false