Why Do I Use Docker?

I am working on a Ruby on Rails app which will soon be up and running on AWS EC2. To my displeasure (as I'm more of an app developer than DevOps guy), I ran into numerous configuration problems setting up my production server. When building Ruby I ran into permissions problems with the gems such as:

ubuntu@ip-172-30-0-78:~$ gem install bundler Fetching: bundler-1.15.0.gem (100%) ERROR: While executing gem ... (Gem::FilePermissionError) You don't have write permissions for the /usr/local/lib/ruby/gems/2.4.0 directory.

Not to mention unforseen missing dependency problems such as this one when building the gemset:

Gem Load Error is: Could not find a JavaScript runtime. See https://github.com/rails/execjs for a list of available runtimes.

I could fiddle around for hours fixing these issues, but then what if I had to upgrade Ruby later on or add new dependencies for new features? The last thing I would want is for my production system to go down due to unforeseen differences between my development environment and production. Docker gives me a way to ensure consistency between environments and swap dependencies.