When you are using containers, a common thing that you will need to do is connect to some external resources such as a database, cache, or web service. These resources usually need credentials. One popular way of passing these credentials to containers is to make them environmental variables that get populated on container startup. This allows you to use the same Docker image for different development environments, and removes the need for storing passwords in the image. This is a common trait for a twelve-factor app (https://12factor.net), which was made popular by Heroku (https://www.heroku.com).
Adding environmental variables to a container at runtime is very easy, but it has its downsides. When an environment variable is added to the container, it is accessible to everything running in that container. That means that, not only can your code...
