Fix CI : mvn introuvable sur le runner (image sans Maven preinstalle)

actions/setup-java installe le JDK mais pas Maven. Ca marche sur les
runners GitHub-hostes (Maven deja present dans l'image), mais pas sur
ce runner self-hoste (act) qui utilise une image minimale. Bascule
sur l'image Docker officielle maven:3.9-eclipse-temurin-17, qui a
Maven + JDK 17 preinstalles, peu importe le runner sous-jacent.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -9,16 +9,18 @@ on:
 jobs:
   build:
     runs-on: ubuntu-latest
+    container: maven:3.9-eclipse-temurin-17
     steps:
       - name: Checkout
         uses: actions/checkout@v4
 
-      - name: Setup JDK 17
-        uses: actions/setup-java@v4
+      - name: Cache Maven dependencies
+        uses: actions/cache@v4
         with:
-          distribution: temurin
-          java-version: '17'
-          cache: maven
+          path: ~/.m2/repository
+          key: ${{ runner.os }}-maven-${{ hashFiles('**/pom.xml') }}
+          restore-keys: |
+            ${{ runner.os }}-maven-
 
       - name: Build and test
         run: mvn -B clean verify