Update website docker version (apache#11546)

### Motivation

Fixed https://github.com/apache/pulsar/runs/3230519747?check_suite_focus=true 

```
/pulsar/pulsar-client-cpp/lib/ProtobufNativeSchema.cc: In function 'pulsar::SchemaInfo pulsar::createProtobufNativeSchema(const google::protobuf::Descriptor*)':
/pulsar/pulsar-client-cpp/lib/ProtobufNativeSchema.cc:51:47: error: 'class google::protobuf::FileDescriptorSet' has no member named 'ByteSizeLong'
     std::vector<char> bytes(fileDescriptorSet.ByteSizeLong());
                                               ^
lib/CMakeFiles/PULSAR_OBJECT_LIB.dir/build.make:1797: recipe for target 'lib/CMakeFiles/PULSAR_OBJECT_LIB.dir/ProtobufNativeSchema.cc.o' failed
make[3]: *** [lib/CMakeFiles/PULSAR_OBJECT_LIB.dir/ProtobufNativeSchema.cc.o] Error 1
make[3]: *** Waiting for unfinished jobs....
make[2]: *** [lib/CMakeFiles/PULSAR_OBJECT_LIB.dir/all] Error 2
CMakeFiles/Makefile2:151: recipe for target 'lib/CMakeFiles/PULSAR_OBJECT_LIB.dir/all' failed
make[1]: *** [python/CMakeFiles/_pulsar.dir/rule] Error 2
make: *** [_pulsar] Error 2
CMakeFiles/Makefile2:310: recipe for target 'python/CMakeFiles/_pulsar.dir/rule' failed
Makefile:227: recipe for target '_pulsar' failed
Error: Process completed with exit code 2.

```

### Modifications

* Update Docker image

site2/tools/docker-build-site.sh

@@ -27,7 +27,7 @@ ROOT_DIR=$(git rev-parse --show-toplevel)
 cd $ROOT_DIR/pulsar-client-cpp
 
 BUILD_IMAGE_NAME="${BUILD_IMAGE_NAME:-apachepulsar/pulsar-build}"
-BUILD_IMAGE_VERSION="${BUILD_IMAGE_VERSION:-ubuntu-16.04-py2-website}"
+BUILD_IMAGE_VERSION="${BUILD_IMAGE_VERSION:-ubuntu-16.04-pb3-website}"
 
 IMAGE="$BUILD_IMAGE_NAME:$BUILD_IMAGE_VERSION"