Here's one benefit you get with docker: Speed of rebuilds, and ensuring that your build instructions and list of dependencies accurately reflects your actual environment. I basically have a "basic dev setup" container for all my projects now, and each new project sits as sub-directories of a directory on the host that I bind mount into the docker containers. Each project then also has a Dockerfile which adds any project specific dependencies.
Building a fresh container then takes a couple of seconds. And the projects run within those containers only. Ever time I restart the apps in my development environment, I rebuild the container, because it is so cheap. Which means I know at any time that the container can be rebuilt to a state the app will run in. I know when I want to deploy that the Dockerfile accurately reflects the dependencies, because otherwise my app wouldn't be running in the dev environment, as any and all changes to anything outside of the application repository are only applied through changes to the Dockerfile.
Sure, but speed of rebuilds is important when you rebuild often, which is not my case.
In my case the Dockerfile is easily replaced with a bash set -e script, which has never gave problems to me: deboostrap, share volumes, aptitude install apache/nginx, mysql/postgresql, php/ruby, copy files, predeployment commands like f.e. bundle install, start services.
And you have the advantage of starting services with `service start ...`, instead of reinventing init scripts.
Building a fresh container then takes a couple of seconds. And the projects run within those containers only. Ever time I restart the apps in my development environment, I rebuild the container, because it is so cheap. Which means I know at any time that the container can be rebuilt to a state the app will run in. I know when I want to deploy that the Dockerfile accurately reflects the dependencies, because otherwise my app wouldn't be running in the dev environment, as any and all changes to anything outside of the application repository are only applied through changes to the Dockerfile.