std::filesystem with GCC 8.3 Docker Image

I have been playing around with Docker and using it to access the latest features of GCC's libstd++

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My docker file using is available here - it grabs the latest official GCC docker image

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https://docdro.id/XIa6Xrp

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And the example it is trying to compile is

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#include <iostream>
#include <filesystem>
namespace fs = std::filesystem;
int main()
{
    std::cout << "Current path is " << fs::current_path() << '\n';
}

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The compiler error I get is

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Step 4/5 : RUN g++ -o hello_world main.cpp -lstdc++fs
 ---> Running in 380d3c669bf9
main.cpp:3:21: error: 'filesystem' is not a namespace-name
 namespace fs = std::filesystem;
                     ^~~~~~~~~~
main.cpp:3:31: error: expected namespace-name before ';' token
 namespace fs = std::filesystem;
                               ^
main.cpp: In function 'int main()':
main.cpp:6:40: error: 'fs' has not been declared
     std::cout << "Current path is " << fs::current_path() << '\n';

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I have confirmed that I have the expect version of compiler

Step 4/5 : RUN g++ --version
 ---> Running in 7bea4b71ec56
g++ (GCC) 8.3.0

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Any suggestions?