[Dockerfile] Eigen repository moved

[313usually]

Eigen moved their repository from Bitbucket to Gitlab recently (the announcement).
So 69-74 lines of the Dockerfile need to be updated like this:

RUN curl -LO https://gitlab.com/libeigen/eigen/-/archive/3.3.4/eigen-3.3.4.tar.gz && \
    tar -zxf eigen-3.3.4.tar.gz && \
    cd eigen-3.3.4 && \
    mkdir build && cd build && \
    cmake .. && make install -j$(nproc) && \
    cd ../../ && rm -fr eigen-3.3.4

[aparrapo]

Thanks for the catch @313usually! You can create a pull request so we can give you proper credit :)