fix(ci): run build-jvm and build-native inside maven container image

Runner has no Maven or JDK installed. Instead of installing
both manually each job, run the jobs inside the official
maven:3.9.6-eclipse-temurin-21 image which has both pre-installed.

- Drop actions/setup-java (image has JDK 21)
- Drop 'Install Maven' step (image has Maven 3.9.6)
- Add 'container: image: maven:3.9.6-eclipse-temurin-21' to each job
- Add cache for ~/.m2/repository (key on pom.xml hash)
- Keep graalvm/setup-graalvm only for the native job (adds GraalVM
  inside the container)
- docker-native job unchanged: uses the runner's docker daemon
  directly to build the image with the pre-built native binary

.gitea/workflows/ci.yml

@@ -17,28 +17,18 @@ jobs:
   build-jvm:
     name: Build JVM
     runs-on: ubuntu-latest
+    container:
+      image: maven:3.9.6-eclipse-temurin-21
     steps:
       - name: Checkout
         uses: actions/checkout@v4
 
-      - name: Set up JDK 21
-        uses: actions/setup-java@v4
+      - name: Cache Maven repository
+        uses: actions/cache@v4
         with:
-          distribution: temurin
-          java-version: 21
-          cache: maven
-
-      - name: Install Maven
-        env:
-          MAVEN_VERSION: 3.9.6
-          MAVEN_HOME: ${{ runner.workspace }}/.maven
-        run: |
-          mkdir -p "$MAVEN_HOME"
-          if [ ! -x "$MAVEN_HOME/bin/mvn" ]; then
-            curl -fsSL "https://archive.apache.org/dist/maven/maven-3/$MAVEN_VERSION/binaries/apache-maven-$MAVEN_VERSION-bin.tar.gz" | tar -xz -C "$MAVEN_HOME" --strip-components=1
-          fi
-          echo "$MAVEN_HOME/bin" >> "$GITHUB_PATH"
-          mvn --version
+          path: ~/.m2/repository
+          key: ${{ runner.os }}-maven-${{ hashFiles('**/pom.xml') }}
+          restore-keys: ${{ runner.os }}-maven-
 
       - name: Cache Node + npm
         uses: actions/cache@v4
@@ -61,28 +51,18 @@ jobs:
   build-native:
     name: Build Native
     runs-on: ubuntu-latest
+    container:
+      image: maven:3.9.6-eclipse-temurin-21
     steps:
       - name: Checkout
         uses: actions/checkout@v4
 
-      - name: Set up JDK 21
-        uses: actions/setup-java@v4
+      - name: Cache Maven repository
+        uses: actions/cache@v4
         with:
-          distribution: temurin
-          java-version: 21
-          cache: maven
-
-      - name: Install Maven
-        env:
-          MAVEN_VERSION: 3.9.6
-          MAVEN_HOME: ${{ runner.workspace }}/.maven
-        run: |
-          mkdir -p "$MAVEN_HOME"
-          if [ ! -x "$MAVEN_HOME/bin/mvn" ]; then
-            curl -fsSL "https://archive.apache.org/dist/maven/maven-3/$MAVEN_VERSION/binaries/apache-maven-$MAVEN_VERSION-bin.tar.gz" | tar -xz -C "$MAVEN_HOME" --strip-components=1
-          fi
-          echo "$MAVEN_HOME/bin" >> "$GITHUB_PATH"
-          mvn --version
+          path: ~/.m2/repository
+          key: ${{ runner.os }}-maven-${{ hashFiles('**/pom.xml') }}
+          restore-keys: ${{ runner.os }}-maven-
 
       - name: Set up GraalVM
         uses: graalvm/setup-graalvm@v1