[coldtea]

Developing/testing for multiple versions at the same time, having repeatable tests, versioning, using the same environment for development as your final deliverable (e.g. a Linux environment not OS X), being able to take your container and run it on production as is, and more things besides?

[manquer]

wouldn't a build server be better for that sort of release testing? like travis CI / gitlab CI runner on DO / light sail etc @ $5/month

[coldtea]

Not all developers have access to a build server or want to bother setting up one.

[barrkel]

Yes; you use the build server to build the docker image, and run tests on the docker image in a test environment.