refactor: improve build script and Dockerfile

- Translate rebuild.sh messages to English
- Add dockerfile syntax directive
- Fix heredoc syntax in Dockerfile

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

gitea_runner_java/Dockerfile

@@ -1,3 +1,4 @@
+# syntax=docker/dockerfile:1
 # Gitea Action Runner Java Image
 # 基于 gitea_runner_base 添加多版本 JDK 和 Maven
 
@@ -40,7 +41,7 @@ ENV JAVA_HOME=${JAVA_BASE}/java-17-openjdk
 ENV PATH="${JAVA_HOME}/bin:${MAVEN_HOME}/bin:${PATH}"
 
 # 创建版本切换脚本
-RUN cat > /usr/local/bin/use-java << 'EOF'
+RUN cat <<'EOF' > /usr/local/bin/use-java
 #!/bin/bash
 case "$1" in
     7)  export JAVA_HOME=/usr/lib/jvm/java-7-zulu ;;