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Patterns for running tests inside a Docker container

'Docker wormhole' pattern - Sibling docker containers

Testcontainers itself can be used from inside a container. This is very useful for different CI scenarios like running everything in containers on Jenkins, or Docker-based CI tools such as Drone.

Testcontainers will automatically detect if it's inside a container and instead of "localhost" will use the default gateway's IP.

However, additional configuration is required if you use volume mapping. The following points need to be considered:

  • The docker socket must be available via a volume mount
  • The 'local' source code directory must be volume mounted at the same path inside the container that Testcontainers runs in, so that Testcontainers is able to set up the correct volume mounts for the containers it spawns.

Docker-only example

If you run the tests with just docker run ... then make sure you add -v $PWD:$PWD -w $PWD -v /var/run/docker.sock:/var/run/docker.sock to the command, so it will look like this:

$ tree .
.
├── pom.xml
└── src
    └── test
        └── java
            └── MyTestWithTestcontainers.java

$ docker run -it --rm -v $PWD:$PWD -w $PWD -v /var/run/docker.sock:/var/run/docker.sock maven:3 mvn test

Where: * -v $PWD:$PWD will add your current directory as a volume inside the container * -w $PWD will set the current directory to this volume * -v /var/run/docker.sock:/var/run/docker.sock will map the Docker socket

Docker Compose example

The same can be achived with Docker Compose:

tests:
  image: maven:3
  stop_signal: SIGKILL
  stdin_open: true
  tty: true
  working_dir: $PWD
  volumes:
    - $PWD:$PWD
    - /var/run/docker.sock:/var/run/docker.sock
    # Maven cache (optional)
    - ~/.m2:/root/.m2
  command: mvn test

Docker-in-Docker

While Docker-in-Docker (DinD) is generally considered an instrument of last resort, it is necessary for some CI environments.

Drone CI is one such example. Testcontainers has a Docker-in-Docker plugin (build image) for use with Drone, which could be used as inspiration for setting up other similar testing using DinD.