docs: introduce documentation and usage for containers

contrib/containers/README.md

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+## Containers
+
+This directory contains configuration files for containerization utilities.
+
+Currently two Docker containers exist, `ci` defines how Dash's GitLab CI container is built and the `dev` builds on top of the `ci` to provide a containerized development environment that is as close as possible to CI for contributors!
+
+### Usage Guide
+
+We utilise edrevo's [dockerfile-plus](https://github.com/edrevo/dockerfile-plus), a syntax extension that
+leverages Docker [BuildKit](https://docs.docker.com/develop/develop-images/build_enhancements/) to reduce
+the amount of repetitive code.
+
+As BuildKit is opt-in within many currently supported versions of Docker (as of this writing), you need to
+set the following environment variables before continuing. While not needed after the initial `docker-compose build`
+(barring updates to the `Dockerfile`), we recommend placing this in your `~/.bash_profile`/`~/.zshrc` or equivalent
+
+```bash
+export DOCKER_BUILDKIT=1
+export COMPOSE_DOCKER_CLI_BUILD=1
+```
+
+After that, it's simply a matter of building and running your own development container. You can use extensions
+for your IDE like Visual Studio Code's [Remote Containers](https://code.visualstudio.com/docs/remote/containers)
+to run terminal commands from inside the terminal and build Dash Core.
+
+```bash
+cd contrib/containers/develop
+docker-compose build
+docker-compose run container
+```