Dockerfile fix for Chromebook

[hellosze]

Hi there,
I am using a Chromebook and I could not run exercism inside the docker container.
I changed the exercism release to the latest one and was able to run exercism.

RUN wget https://github.com/exercism/cli/releases/download/v3.1.0/exercism-3.1.0-linux-armv6.tar.gz && \
    tar xzf exercism-3.1.0-linux-armv6.tar.gz && \
    mv exercism /usr/local/bin/