Stop exposing BUNDLER_VERSION and RUBYGEMS_VERSION environment variables into containers

It is a convenient but not required feature and it was causing the following cryptic error in the binstub generated by Bundler itself (but not in the binstub generated by Rubygems) when `BUNDLER_VERSION` was an empty string:
```
/usr/local/lib/ruby/site_ruby/3.0.0/rubygems/requirement.rb:106:in `parse': Illformed requirement [""] (Gem::Requirement::BadRequirementError)
	from /usr/local/lib/ruby/site_ruby/3.0.0/rubygems/requirement.rb:138:in `block in initialize'
	from /usr/local/lib/ruby/site_ruby/3.0.0/rubygems/requirement.rb:138:in `map!'
	from /usr/local/lib/ruby/site_ruby/3.0.0/rubygems/requirement.rb:138:in `initialize'
	from /bundle/bin/bundle:92:in `new'
	from /bundle/bin/bundle:92:in `activate_bundler'
	from /bundle/bin/bundle:81:in `load_bundler!'
	from /bundle/bin/bundle:105:in `<top (required)>'
	from /bundle/bin/whenever:17:in `load'
	from /bundle/bin/whenever:17:in `<main>'
```

Author: michaelbaudino

Dockerfile

@@ -14,9 +14,7 @@ ARG RUBYGEMS_VERSION_ARG="" \
     BUNDLER_VERSION_ARG=""
 
 # Define dependencies base versions
-ENV RUBYGEMS_VERSION=${RUBYGEMS_VERSION_ARG:-${RUBYGEMS_VERSION}} \
-    BUNDLER_VERSION=${BUNDLER_VERSION_ARG} \
-    NODE_VERSION="16" \
+ENV NODE_VERSION="16" \
     GOSU_VERSION="1.16"
 
 # Define some default variables
@@ -164,8 +162,8 @@ RUN set -eux; \
 
 # Install GEM dependencies
 # Note: we still need Bundler 1.x because Bundler auto-switches to it when it encounters a Gemfile.lock with BUNDLED WITH 1.x
-RUN gem update --system ${RUBYGEMS_VERSION} \
- && gem install bundler${BUNDLER_VERSION:+:${BUNDLER_VERSION}} \
+RUN gem update --system ${RUBYGEMS_VERSION_ARG} \
+ && gem install bundler${BUNDLER_VERSION_ARG:+:${BUNDLER_VERSION_ARG}} \
  && gem install bundler:1.17.3
 
 # Add dot files to the home directory skeleton (they persist IRB/Pry/Rails console history, configure Yarn, etc…)

RECIPES.md

@@ -431,7 +431,7 @@ This can be particularly useful with configurations like the one traditionally s
 
 ### Bundler 1.x
 
-Our image uses Bundler 2.x by default, but for convenience, it also embeds Bundler 1.x. To use it in your legacy project, simply set the `BUNDLER_VERSION` environment variable from your `docker-compose.yml`:
+Our image uses Bundler 2.x by default, but for convenience, it also embeds Bundler 1.x. To use it in your legacy project, simply set the [(undocumented) `BUNDLER_VERSION` environment variable](https://github.com/rubygems/rubygems/issues/3978) from your `docker-compose.yml`:
 
 ```yaml
 services:

test/cowsay/Gemfile.lock

@@ -1,7 +1,7 @@
 GEM
   remote: https://rubygems.org/
   specs:
-    ruby_cowsay (0.1.2)
+    ruby_cowsay (0.1.3)
 
 PLATFORMS
   ruby
@@ -10,4 +10,4 @@ DEPENDENCIES
   ruby_cowsay
 
 BUNDLED WITH
-   1.16.6
+   2.4.17