Replace ADD in Dockerfile to follow Docker best practices

[haxorof]

ADD today fetches the release here from Github but according to Docker best practices this kind of use of ADD shall be avoided:

• https://docs.docker.com/engine/userguide/eng-image/dockerfile_best-practices/#add-or-copy

Construct similar to this shall be used instead:

RUN mkdir -p /usr/src/things \
    && curl -SL http://example.com/big.tar.xz \
    | tar -xJC /usr/src/things \
    && make -C /usr/src/things all