Work out how to reduce the image size

[jsierles]

The image produced by the current setup weighs 340MB. This isn't terrible for a typical Rails image, but could be a lot smaller based on what's visible in the image.

Using dive we see a large 245MB layer at the bottom. This layer contains the source code, rubygems, and the rubygems dependencies. This means that every code change will require pushing a 245MB layer. Let's see how we can improve this.

Here are tickets describing specific options to explore:

• Update the pg gem in Nixpkgs to depend on postgresql.lib to reduce the closure size #6
• Skip inclusion of gems in the development/test groups #8
• Customize the layering strategy #9 and Look at strategies to separate runtime vs. app in containerization layers #7
• Increase the layer count #10

[jsierles]

Here's relevant related issue covering just the pg gem issue: NixOS/nixpkgs#61580

[jsierles]

We can open more specific tickets as needed for size optimization.