Add Dockerfile and Gitea CI/CD workflows

- Dockerfile: eclipse-temurin:25-alpine, exposes port 8080
- build.yml: builds on every branch push with Java 25
- deploy.yml: builds and pushes Docker image on v* tags

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.gitea/workflows/build.yml

@@ -0,0 +1,21 @@
+name: Build
+on:
+  push:
+    branches:
+      - '*'
+jobs:
+  build:
+    runs-on: standard-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Java
+        uses: actions/setup-java@v4
+        with:
+          distribution: 'temurin'
+          java-version: '25'
+          cache: 'gradle'
+
+      - name: Build
+        run: ./gradlew build --no-daemon