[website] fix: website build fail due to java home setting wrong in docker container (#13586)

Signed-off-by: LiLi <urfree@apache.org>
diff --git a/site2/tools/docker-build-site.sh b/site2/tools/docker-build-site.sh
index ca1ddce..7028291 100755
--- a/site2/tools/docker-build-site.sh
+++ b/site2/tools/docker-build-site.sh
@@ -47,4 +47,4 @@
 sed -i "s#$ROOT_DIR#/pulsar#g" $ROOT_DIR/distribution/server/target/classpath.txt
 sed -i "s#$HOME#/root#g" $ROOT_DIR/distribution/server/target/classpath.txt
 
-$DOCKER_CMD bash -l -c 'cd /pulsar && /pulsar/site2/tools/build-site.sh'
+$DOCKER_CMD bash -l -c 'export JAVA_HOME=/usr/lib/jvm/java-11-openjdk-amd64 && cd /pulsar && /pulsar/site2/tools/build-site.sh'