Provide a way to compile node-sass binding

[shouze]

In #40 the binding is added into the container directly, but maybe that when it's needed to upgrade this binary (alpine bin upgrades or node abi BC break), would be nice to provide a way to compile again this binding into the container.

I guess this would be possible using --build-arg of docker build command and conditionnaly launch the following kind of sequence:

RUN mkdir fake project && cd fake-project \
      && yarn add node-sass \
      && cp node_modules/vendor/...path_to_the_bin /home/node/the-compiled-bin

Also, the compiled bin produced into the container should also be put down in the repo this way when the build arg is not provided we still copy from the git repo into the container we build.

[shouze]

Ok maybe official binaries will be there soon! sass/node-sass#1589 (comment)

[shouze]

ok done in #45, node-sass now maintain a binary binding for alpine/musl 🎉