Watir/Firefox in Docker via Xvfb
https://hub.docker.com/r/mipmip/watir-xvfb/
This Docker image provides a way to run headless Watir tests with Firefox or actually Iceweasel because we're running Debian. I borrowed the xvfb initializer from markadams/chromium-xvfb.
Running the sample test
docker run watir ruby /app/test.rb