Add section about Ruby image assumptions

[tianon]

Closes docker-library/ruby#129
Closes docker-library/ruby#83
Closes docker-library/ruby#191
Closes docker-library/ruby#188

I've verified that this recommendation works by taking the Dockerfile provided in docker-library/ruby#188 (comment) and adding ENV BUNDLE_BIN= (to unset BUNDLE_BIN):

FROM ruby:2.4

ENV BUNDLE_BIN=

RUN set -ex;\
    git clone https://github.com/jwt/ruby-jwt.git;\
    cd ruby-jwt;\
    bundle install;\
    cd ..;\
    git clone https://github.com/janlelis/clipboard.git;\
    cd clipboard;\
    bundle install;\
    ls -la /clipboard/Gemfile.lock

$ docker build --pull .
Sending build context to Docker daemon  2.048kB
...
Step 2/3 : ENV BUNDLE_BIN=
...
Step 3/3 : RUN set -ex;    git clone https://github.com/jwt/ruby-jwt.git;    cd ruby-jwt;    bundle install;    cd ..;    git clone https://github.com/janlelis/clipboard.git;    cd clipboard;    bundle install;    ls -la /clipboard/Gemfile.lock
...
+ ls -la /clipboard/Gemfile.lock
-rw-r--r-- 1 root root 661 May 11 20:54 /clipboard/Gemfile.lock
Removing intermediate container fbd54d26c43b
 ---> 399ec4e9f562
Successfully built 399ec4e9f562