Fixed broken link to Singularity source download by including it in the repo.

ENV GOPATH=/root/go

ENV PATH=$PATH:/usr/local/go/bin:$GOPATH/bin

COPY singularity-3.4.1.tar.gz /tmp

# Install Singularity

RUN mkdir -p /usr/local/var/singularity/mnt && \

    mkdir -p $GOPATH/src/github.com/sylabs && \

    cd $GOPATH/src/github.com/sylabs && \

    wget https://github.com/sylabs/singularity/releases/download/v3.4.1/singularity-3.4.1.tar.gz && \

    mv /tmp/singularity-3.4.1.tar.gz ./ && \

    tar -xzvf singularity-3.4.1.tar.gz

RUN cd $GOPATH/src/github.com/sylabs/singularity && \

    ./mconfig -p /usr/local && \