libatomic.so.1 needed in /runtime for RISC-V about dart-docker

[cpswan]

It seems that libatomic.so.1 has been removed from the debian:unstable image for linux/riscv64 but it's still needed by dart compile exe (testing with 3.2.0-42.1.beta).

To deal with this I've modified the Dockerfile to install libatomic1:

RUN set -eux; \
    apt-get update; \
    apt-get install -y --no-install-recommends \
        ca-certificates \
        curl \
        dnsutils \
        git \
        libatomic1 \
        openssh-client \
        unzip \
    ; \
    rm -rf /var/lib/apt/lists/*

and then added the library to the list that gets copied into runtime:

        riscv64) \
            TRIPLET="riscv64-linux-gnu" ; \
            FILES="/lib/ld-linux-riscv64-lp64d.so.1 \
                /lib/riscv64-linux-gnu/ld-linux-riscv64-lp64d.so.1 \
                /usr/lib/riscv64-linux-gnu/libatomic.so.1" ;; \