Support for micro versions of Ruby

[dsyer]

I needed to use Ruby 2.7.4 for a specific project that has that hard-coded in the Gemfile. Devcontainers standard containers only let me specify the minor version (2.7) and that lands on 2.7.6 currently, so my project won't build.

[bamurtaugh]

cc @jarrodldavis for thoughts, as one of the contributors listed on https://github.com/microsoft/vscode-dev-containers/tree/main/containers/ruby-rails-postgres.

[simensol]

This would also be a great feature for other languages, too (read: Python ❤️).

[eitsupi]

It may be useful to have an option to chose the base.Dockerfile, so users can freely change the base image. https://github.com/microsoft/vscode-dev-containers/blob/ab35cd690c2e357379bcebae8303d0c8704b8882/containers/ruby/.devcontainer/base.Dockerfile#L3