Investigating Docker

I am investigating using Docker with my existing CI app. What concerns me (from my reading) is that Docker is "environment-agnostic", a part of which seems to be  setting a container to read only. I plan on using 3 Bitnami containers (as per the bitnami documentation), one for Apache, one for php-fpm, and one for MariaDB. What I am concerned about is the way that CI writes out to files to keep track of things such as session variables (actually i don't remember the details but I do remember that something gets written out). 

My plan is to set up a development environment on my Laptop for development and testing. Debugging on the server almost "killed me". Anyone done any investigation into using Docker?

proof that an old dog can learn new tricks