Connecting to a C++ websocketpp server within Docker

Question

I am trying to establish a websocket connection with a server written in C++ with websocketpp. I can successfully establish a connection with my server when it is running on my computer, but I am unable to connect with the same server when it is running in a Docker container on my local machine.

I spent a while looking up this issue online, and it seemed that the main pain points were to listen to 0.0.0.0 instead of 127.0.0.1, and to use the -p config instead of EXPOSE in the Dockerfile. I think what I have now follows those two pieces of advice, but I am still unable to connect to the endpoint. I was pretty lost, so I figured I would ask here to see if anyone else ran into a similar issue.

My client looks like this (just from within the browser):

let ws = new WebSocket('ws://localhost:3001');

My server looks like this:

websocketpp::server<websocketpp::config::asio> server_;

server_.clear_access_channels(
   websocketpp::log::alevel::frame_header | websocketpp::log::alevel::frame_payload
); 
server_.set_reuse_addr(true);
server_.init_asio();

server_.listen(port_); // port_ is set to 3001

server_.start_accept();
server_.run();

and my Dockerfile looks like the following:

FROM gcc:latest

RUN set -ex; \
    apt-get -y update; \
    apt-get -y upgrade; \
    apt-get install -y cmake build-essential libboost-dev \
        libboost-system-dev libboost-thread-dev librange-v3-dev \
        libfmt-dev libprotobuf-dev protobuf-compiler \
        libwebsocketpp-dev libgtest-dev google-mock

# libgtest-dev only installs source files which need to be compiled -- actually compile them here
RUN cd /usr/src/googletest; \ 
    cmake .; \
    cmake --build . --target install 

COPY . /usr/src/mazer
WORKDIR /usr/src/mazer

RUN cd /usr/src/mazer;  \
    mkdir build;  \
    cd build; \
    cmake ..; \
    make

CMD exec "./build/mazer-server/mazer-server"

I am starting the container with the following command:

docker run --rm -p 3001:3001 mazer-server