Install Node.js and Yarn quickly in docker files

The install_node.sh script is a high-performance script for setting up Node & Yarn in docker, it is fast, secure and more efficient than regular installation methods, taking advantage of the way docker works.

Why it is better? ⚡️

You reduce the lines in your docker file to 1
It downloads both Yarn and Node in parallel - fast
It securely supports mirrors in case you have a faster mirror to download Node or Yarn.
Every variable is taken directly.
It doesn't leave any files behind - the Docker image will not hold any temporary files
It removes even npm & docs files to make the image even slimmer

Installation Instructions 🚀

Be sure that you have all dependencies (debian example):
```
RUN apt-get update \
    && apt-get install -y \
          build-essential \
          git \
          curl \
    && rm -rf /var/cache/apk/*
```
Note: build-essential is not necessarily required, but if any node build process requires a c, c++ or other library, the yarn install process might fail.
Either A or B

A. Copy this file in your project and add following to your Dockerfile
```
ADD install_node.sh
RUN NODE_VERSION="v5.1.0" \
    YARN_VERSION="v0.19.1" \
    bash /install_node.sh \
    && rm scripts/install_node.sh
```
This is better when you want to reduce the build time and are okay with updating this script by hand.

B. Use curl to download the file from github:
```
RUN curl -sL https://raw.githubusercontent.com/martinheidegger/install-node/master/install_node.sh  | \
    NODE_VERSION="v5.1.0" \
    YARN_VERSION="v0.19.1" \
    bash
```
This is better when you want to make sure that this build file is up-to-date and to reduce the dockerfile steps.

Specifying a node mirror

If you want to use a different mirror to download node or yarn you can specify it like:

RUN NODE_MIRROR="https://nodejs.org/dist" \
    YARN_MIRROR="https://myyarnmirror.org/dist" \
    NODE_VERSION="v5.1.0" \
    YARN_VERSION="v0.19.1" \
    (curl -sL /install_node.sh | bash)

Note: Even though you specify a different mirror, the integration checks will run against data from the original mirrors!

Installation variants

By default this script will take the 'linux-x64' variant. If you need another variant, specify it like

RUN NODE_VARIANT="linux-x86" \
    NODE_VERSION="v5.1.0" \
    YARN_VERSION="v0.19.1" \
    (curl -sL /install_node.sh | bash)

You can peek into a node dist folder like https://nodejs.org/dist/v6.9.4/ to look for the available variants.

Special variant "make"

If you supply the NODE_VARIANT="make" it will download the sources and install them directly, this is important in case the binary doesn't work in operating systems such as alpine-linux.

Usable node

The default node script is trimmed for production, which means files like docs or npm will be dropped! In order To keep them it you can pass in: KEEP_EXTRAS=true and all the nice files will be kept.