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The mysql-server install step is now obsolete. It needs to install mariadb-server.

sudo apt-get install mariadb-server mariadb-client libmariadb-dev-compat libmariadb-dev

I was able to follow rest of the steps and import XF DB, so someone should update the guide.

Now I am stuck on this following step and need help.

echo "gem 'mysql2'" >>Gemfile
bundle install --no-deployment

Running the above gives me the following error. I checked the Gemfile and it only contains this one line - gem ‘mysql2’

This Gemfile does not include an explicit global source. 
Not using an explicit global source may result in a different lockfile being generated depending on the gems you have installed locally before bundler is run. 
Instead, define a global source in your Gemfile like this: source "https://rubygems.org".
Could not find gem 'mysql2' in locally installed gems.
root@ip-172-566-459-13-app:/#