Add some "gem install" tests (from the most popular gems list)

test/config.sh

@@ -18,6 +18,7 @@ declare -A imageTests=(
 		python-zlib
 	'
 	[ruby]='
+		ruby-gems
 		ruby-standard-libs
 	'
 # example onbuild

test/tests/ruby-gems/container.sh

@@ -0,0 +1,20 @@
+#!/bin/bash
+set -e
+
+# list taken from https://rubygems.org/stats
+gems=(
+	thor
+	rake
+	rails
+	rack
+	activesupport
+	activerecord
+	actionpack
+	json
+	actionmailer
+	activeresource
+)
+
+for gem in "${gems[@]}"; do
+	gem install "$gem"
+done

test/tests/ruby-gems/run.sh

@@ -0,0 +1 @@
+../run-bash-in-container.sh