Build container image in CI, bake in datasets, configure auth via env

- Dockerfile bakes the app and every CSV in data/ into an nginx image
- UI discovers data/*.csv via nginx JSON autoindex and offers a
  dropdown when there is more than one dataset
- docker-entrypoint.d/40-basic-auth.sh creates .htpasswd from
  BASIC_AUTH_USER / BASIC_AUTH_PASSWORD(_FILE) and refuses to start
  without credentials; replaces set-password.sh
- Gitea Actions workflow builds and pushes
  registry.traberph.de/public/interval_viz (latest, sha-*, semver)
- Escape resets the chart zoom

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitea/workflows/docker.yml

@@ -0,0 +1,47 @@
+name: Build image
+
+on:
+  push:
+    branches: [main]
+    tags: ["v*"]
+  pull_request:
+  workflow_dispatch:
+
+env:
+  REGISTRY: registry.traberph.de
+  IMAGE: registry.traberph.de/public/interval_viz
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: docker/setup-buildx-action@v3
+
+      - name: Log in to registry
+        if: github.event_name != 'pull_request'
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ secrets.REGISTRY_USERNAME }}
+          password: ${{ secrets.REGISTRY_TOKEN }}
+
+      - name: Image tags
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.IMAGE }}
+          tags: |
+            type=raw,value=latest,enable={{is_default_branch}}
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=sha,format=short
+
+      - 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 }}