fix: install Maven 3.9.9 in docker build job — same fix as ci.yml

actions/setup-java@v4 installs the JDK but not Maven itself.
pi-runner does not have mvn pre-installed. Add an explicit curl
install step matching the approach used in backend/Dockerfile.

.gitea/workflows/docker.yml

@@ -81,6 +81,15 @@ jobs:
           java-version: '21'
           cache: maven
 
+      # actions/setup-java installs the JDK but not Maven.
+      # pi-runner has no pre-installed mvn — download the same version used
+      # in backend/Dockerfile to keep the build environment consistent.
+      - name: Install Maven 3.9.9
+        run: |
+          curl -fsSL https://archive.apache.org/dist/maven/maven-3/3.9.9/binaries/apache-maven-3.9.9-bin.tar.gz \
+            | tar -xz -C /opt
+          echo "/opt/apache-maven-3.9.9/bin" >> $GITHUB_PATH
+
       - name: Build JAR (native — avoids QEMU Maven download)
         working-directory: backend
         run: mvn -q -ntp clean package -DskipTests
@@ -143,7 +152,7 @@ jobs:
               docker compose ps
             '
 
-  # ── Build failure notification ───────────────────────────────────────────────
+  # ── Build failure notification ──────────────────────────────────────────────
   notify-failure:
     name: Notify — build failure
     runs-on: ubuntu-latest