SURVIVING ENVIRONMENT HELL WITH DOCKER
Docker provides reproducible environments across many platforms so software can
run as expected.

It abstracts away package managers, network interfaces and so much more. I guess
Docker can be viewed as some sort of operating system.

ARCHITECTURE

The Docker ecosystem is structured like so:

A repository holds many Docker images; a registry serves Docker images; a Docker
image is essentially a file system and configuration telling the Docker binary
how to set things up for the environment; a container is a runtime instance of
the image (like a class vs object).

An image is composed of layers, which are just differences in the file system
recorded. The differences that it watches for are done by the ADD/COPY/RUN
commands.

OPERATIONS

Docker operates on images and containers, so these commands will be common:

* `docker info`
* `docker build -t <tag name>` to build an image.
* `docker run <tag name>` to run an image.
* `docker image <operation>`
* `docker container <operation>`

`docker login` is useful for getting access to other remote registries other
than Docker Hub (Docker Company's registry).

ENVIRONMENT CONFIGURATIONS

To configure an environment, a `Dockerfile` is used, which is simple for the
most part. Below is an example of the most typical commands you'd use inside
one:

  # Use an official Python runtime as a parent image
  # FROM *MUST* be the first command in all Dockerfiles.
  FROM python:2.7-slim

  # You can use a MAINTAINER command too, for support contact
  MAINTAINER inbox@leefallat.ca

  # Set the working directory to /app
  WORKDIR /app

  # Copies between the user's file system and the image.
  # Copy the current directory contents into the container at /app
  COPY . /app

  # There is also an ADD command, and it works like COPY but it is 100x more
  # powerful and so it is somewhat dangerous to use. It can copy things from
  # remote sources and does unpacking and probably other things. Review the
  # documentation before using it.

  # Runs arbitrary commands for setup. RUN will commit file system changes to the
  # image file.
  # Install any needed packages specified in requirements.txt
  RUN pip install --trusted-host pypi.python.org -r requirements.txt

  # For networking - very easy as you see!
  # Make port 80 available to the world outside this container
  EXPOSE 80

  # Define an environment variable
  ENV NAME World

  # There *MUST* only be one CMD command in a Dockerfile.
  # This is essentially to start your software.
  # Run app.py when the container launches
  CMD ["python", "app.py"]

  # You can also use ENTRYPOINT which is a bit better - it lets you pass
  # parameters to your program within the container. :) You can only have one of
  # these too.