Docker

Docker is a way of deploying apps on servers. It packages apps into side-by-side containers. These are similar in spirit to virtual machines, but different because containers are not (or, don't have to be) fully bundled operating systems. The container system makes sure everything runs the same everywhere.

It's like Homebrew or Aptitude for apps.

Security

How secure are docker containers, really? https://blog.docker.com/2013/08/containers-docker-how-secure-are-they/

References

Docker's documentation is excellent: https://docs.docker.com/

Docker command line: https://docs.docker.com/engine/reference/commandline/cli/#environment-variables

Docker run: https://docs.docker.com/engine/reference/commandline/run/#full-container-capabilities---privileged

Docker to run memcached (Digital Ocean guide): https://www.digitalocean.com/community/tutorials/docker-explained-how-to-create-docker-containers-running-memcached

Note that many Docker issues on Github contain a wealth of useful debugging strategies and commands for checking the state of networks, ports, etc.: https://github.com/docker/docker/issues/13914