Iterating locally using Docker

For most infrastructure code, your only option is to deploy into a real environment such as AWS. However, if you’re writing scripts (i.e., Bash, Python, or Go), you should be able to test them locally using Docker. Docker containers typically build 10x faster and start 100x faster than real servers, so using Docker for testing can help you iterate much faster.

Here are some techniques we use with Docker:

• If your script is used in a Packer template, add a Docker builder to the template so you can create a Docker image from the same code. See the Packer Docker Example for working sample code.

• We have prebuilt Docker images for major Linux distros that have many important dependencies (e.g., curl, vim, tar, sudo) already installed. See the test-docker-images folder for more details.

• Create a docker-compose.yml to make it easier to run your Docker image with all the ports, environment variables, and other settings it needs. See the Packer Docker Example for working sample code.

• With scripts in Docker, you can replace some real-world dependencies with mocks! One way to do this is to create some “mock scripts” and to bind-mount them in docker-compose.yml in a way that replaces the real dependency. For example, if your script calls the aws CLI, you could create a mock script called aws that shows up earlier in the PATH. Using mocks allows you to test 100% locally, without external dependencies such as AWS.