Add Docker deployment

[artemmukhin]

No description provided.

[Undin]

As I understand, volumes here are needed to be able to apply changes made locally without rerunning configuration. But why only _posts and assets/posts/ directories? What if I want to change some other places? Maybe it's better to add volume for the whole project directory instead of copying its files during docker image building?

[artemmukhin]

The trick here is to preserve Gemfile and Gemfile.lock that were patched during the image build. I couldn't find a better way to do that, but if you know one – please share it.

As we can see at https://pages.github.com/versions, GitHub uses Ruby 2.7.4. But I cannot deploy the website locally using this Ruby version on my ARM-based machine because of some dependencies incompatibilities. That's why my Dockerfile uses a newer Ruby version and thus has to patch the gem dependencies to make it work.

I've extended the list of directories: _posts/, assets/, _docs/, _includes/. They seem to be the most important directories for changes to be auto-applied.

And by @vlad20012 advice, I've added all of them to .dockerignore not to do the useless job, and I've also fixed the Ruby version to avoid incompatibilities in the future.

[artemmukhin]

@vlad20012 Thanks for the careful review!