Docker

Docker is a platform for development and deployment with containers. Containers provide similar isolation to virtual machines but use less computing power by leveraging the host kernel. Docker enables standardized packaging. Applications become portable as they are abstracted from the environment in which they are run.

Information on this page is taken from Docker's getting started guide.

Concepts

An image is an executable package that includes everything needed to run an application - the code, a runtime, libraries, environment variables, and configuration files. A container is a runtime instance of an image - what the image becomes in memory when executed (a process).

Execution

Run a Docker image with docker run <image>. The -p argument is often used to map a machine's port to a container's published port. Specify -d to run the container in the background with detached mode.

List all running containers with docker ps, include --all to also show stopped containers.

Type docker stop <container> to stop the container. You may also kill and rm containers. Lastly, run docker container prune to remove all stopped containers.

Dockerfile

A Dockerfile is a plain text file that contains a list of commands to create a Docker image. An example appears below. Build the Docker image with docker build .. The -t (tag) argument is often used to name the image. 

# Define a parent image.
FROM python:3.6-slim

# Set the working directory.
WORKDIR /app

# Copy to the container.
COPY . /app

# Execute commands.
RUN pip install -r requirements.txt

# Make ports available externally to the container.
EXPOSE 80

# Define environment variables.
ENV NAME World

# Run commands on launch.
CMD ["python", "app.py"]

Registries

A registry is a collection of repositories, and a repository is a collection of images. Run docker pull <image> to fetch an image from a registry. See all downloaded images with docker images. Official images are maintained and supported by Docker. They are typically one word long. User images build on base images are usually formatted as user/image-name.

A raw docker build will build and place an image on your machine's local registry. Publish images to an external registry by logging in (docker login) and running docker push <image>. Images can be tagged with docker tag <image> <name>. Associate a local image with a repository on a registry with the notation username/repository:tag. For example, docker tag example elliot/experiments:1.0.

TODO docker-compose