Migrating to docker compose

Even with a simple image, we've already been dealing with plenty of command line options in both building, pushing and running the image.

Next we will switch to a tool called docker-compose to manage these.

docker-compose is designed to simplify running multi-container applications to using a single command.

In the folder where we have our Dockerfile with the following content:

FROM ubuntu:18.04

WORKDIR /mydir

RUN apt-get update
RUN apt-get install -y curl python
RUN curl -L https://yt-dl.org/downloads/latest/youtube-dl -o /usr/local/bin/youtube-dl
RUN chmod a+x /usr/local/bin/youtube-dl

ENV LC_ALL=C.UTF-8

ENTRYPOINT ["/usr/local/bin/youtube-dl"]

we create a file called docker-compose.yml:

version: '3'

services:
    youtube-dl-ubuntu:
      image: <username>/<repositoryname>
      build: .

The version setting is not very strict, it just needs to be above 2 because otherwise the syntax is significantly different. See https://docs.docker.com/compose/compose-file/ for more info. The key build: value can be set to a path (ubuntu), have an object with context and dockerfile keys or reference a url of a git repository.

Now we can build and push with just these commands:

$ docker-compose build
$ docker-compose push

Volumes in docker-compose

To run the image as we did previously, we will need to add the volume bind mounts. Volumes in docker-compose are defined with the following syntax location-in-host:location-in-container. Compose can work without an absolute path:

version: '3.5'

services:

    youtube-dl-ubuntu:
      image: <username>/<repositoryname>
      build: .
      volumes:
        - .:/mydir
      container_name: youtube-dl

We can also give the container a name it will use when running with container_name. And the service name can be used to run it:

$ docker-compose run youtube-dl-ubuntu https://imgur.com/JY5tHqr

Web services in docker-compose

Compose is really meant for running web services, so let's move from simple binary wrappers to running a HTTP service.

https://github.com/jwilder/whoami is a simple service that prints the current container id (hostname).

$ docker container run -d -p 8000:8000 jwilder/whoami
  736ab83847bb12dddd8b09969433f3a02d64d5b0be48f7a5c59a594e3a6a3541

Navigate with a browser or curl to localhost:8000, they both will answer with the id.

Take down the container so that it's not blocking port 8000.

$ docker container stop 736ab83847bb
$ docker container rm 736ab83847bb

Let's create a new folder and a docker-compose file whoami/docker-compose.yml from the command line options.

version: '3.5'

services:
    whoami:
      image: jwilder/whoami
      ports:
        - 8000:8000

Test it:

$ docker-compose up -d
$ curl localhost:8000

Environment variables can also be given to the containers in docker-compose.

version: '3.5'

services:
  backend:
      image:
      environment:
        - VARIABLE=VALUE
        - VARIABLE