Add ENV variables for node and bundler version

[giladshanan]

First of all, thank you @robertkaczmarek for this image. I have been using it for testing via Github Actions and finding it very useful.

I'm wondering if you'd be open to adding ENV variables to allow easily setting the desired version of node and bundler to build. This would allow easy configuration for testing or other uses. You could set a default value to keep existing behavior, something like:

RUN curl -sL https://deb.nodesource.com/${NODE_VERSION:-setup_10.x} | bash -
...
RUN gem install bundler -v ${BUNDLER_VERSION:-1.17.3} --no-document

I'm happy to open a PR if there are no major concerns, let me know what you think.