build(deploy): Add configurable Maven mirror

Add MAVEN_MIRROR_URL for 1Panel backend builds so deployments can
use a faster regional mirror or Maven Central based on network access.

Update the 1Panel deployment guide to document the mirror option and
keep first-time Nacos credentials guidance near environment setup.

backend/Dockerfile.service.1panel

@@ -7,7 +7,24 @@ WORKDIR /workspace
 COPY . .
 
 ARG MODULE
-RUN mvn -pl "${MODULE}" -am package -Dmaven.test.skip=true
+ARG MAVEN_MIRROR_URL=https://maven.aliyun.com/repository/public
+
+RUN mkdir -p /root/.m2 && cat > /root/.m2/settings.xml <<EOF
+<settings xmlns="http://maven.apache.org/SETTINGS/1.2.0"
+          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+          xsi:schemaLocation="http://maven.apache.org/SETTINGS/1.2.0 https://maven.apache.org/xsd/settings-1.2.0.xsd">
+  <mirrors>
+    <mirror>
+      <id>mirror</id>
+      <mirrorOf>*</mirrorOf>
+      <name>mirror</name>
+      <url>${MAVEN_MIRROR_URL}</url>
+    </mirror>
+  </mirrors>
+</settings>
+EOF
+
+RUN mvn -s /root/.m2/settings.xml -pl "${MODULE}" -am package -Dmaven.test.skip=true
 
 FROM eclipse-temurin:17-jre