Install docker only if we need too
Overview
In !29 (merged) we added support for our cookbook to install Docker because some images don't come with Docker preinstalled. We specify a environment variable, which is less then ideal because the developer has to know/remember when to specify this variable if they want to install Docker.
Proposal
Update our docker recipe to first check if docker is installed or not. If it is installed it will exit early, if it's not installed it will continue with installing it.
The benefits we get:
- We don't depend on an environment variable that the user has to set.
- It's automatic
- Upgrading the docker version can be much easier because we would only have to update 1 file instead of multiple declarations of INSTALL_DOCKER_VERSION