add custom docker image that includes the gem dependencies to speed up make serve

[andynog]

When running the command make build, there's a step that makes it run longer which is installing the gem dependencies from the Gemfile. One idea is to create a custom docker image based on the one we currently use jekyll/builder and use the custom one to host the website locally.

There might be also ways to re-use the gem dependencies downloaded during make build but I couldn't find a good solution.

The process to build this custom docker image could be also be included in the Github Actions to ensure a new image is always up to date.