Feature Request: Test Basic Gem Installation on Images

**Describe the Feature Request**
Currently it appears no tests run on any images to ensure gems install properly / ruby is configured. Suggest having a basic test run to install a list of common gems ensuring ruby is properly installed `gem install bcrypt` / `gem install rails` / `gem install pg` (fairly common gems).

**Is your feature request related to a particular problem?**
The 3.3 images were published without jemalloc installed causing issues in installing all gems (see: https://github.com/CircleCI-Public/cimg-ruby/pull/156).

**How will this feature request benefit CircleCI jobs using this image?**
This'll ensure the images work when published.

**Describe the solution you would like to see**
Run the tests on building the images.

**Describe alternatives you have considered**
N/A

No files or test entry points are named in the issue. Start by inspecting the image build and test workflow, then determine where the listed bcrypt, rails, and pg installations can run; done means image builds test those gem installations successfully before publication.

Written by the indexing model from the issue text.