[anschwa]

Great question! In my experience with ruby on rails, the application runtime is so dynamic, that even unit tests may not be reliable if run outside the production image.

I think normally this isn't much of an issue because other automated/manual integration testing in the staging environments will catch any major problems, to your point.

Another example would be for browser testing via chromedriver. I've usually seen this implemented along side unit tests (i.e., prior to the build phase) but since it generally serves as an integration level test for many applications, this has lead to issues due to the testing and production environments being out of sync.

I think multi-layer Docker images are a compelling solution to this, but it's not usually how I've seen it implemented.

Instead, I've typically seen that test and prod environments are manually maintained. Sometimes these are two separate Dockerfiles or some shared CI environment managed by different teams.

[happymellon]

> Another example would be for browser testing via chromedriver.

That shouldn't be in your master image. Put your application in a container, and either just run your browser tests and point it at your dev/test/prod deployment, or build a second image with Chrome driver and the test scripts and point it at your deployed application.

Doesn't need to be multi-layer, and it doesn't need to be complex. You have essentially two apps here.