add requirement to install docker

tests/README.md

@@ -9,6 +9,7 @@ Setting up an environment for testing is quite simple.
 
 From a clean Linux image you run the following commands, and you'll be all set.
 
+0. install docker (perhaps other packages?)
 1. Download the Sarracenia source  
   `git clone https://github.com/MetPX/sarracenia`
 2. Checkout the branch you're working on (optional)  
@@ -107,4 +108,4 @@ If the code isn't already on your system, then the following should get you setu
   `docker run --rm -it --name sr3-pytest ubuntu bash`
 - Install dependencies (Ubuntu/Debian)  
   `apt-get update && apt-get install -y git python3 python3-pip`
-- Then follow the Setup directions above
+- Then follow the Setup directions above