In Three Lines#

• In the development of a Rails application, I prefer a setup where the Rails app runs only on the host machine, while middleware like MySQL and Redis runs in Docker containers (Docker for Mac).
• When trying to use MiniMagick in a Rails app, it is necessary to install ImageMagick in the environment running the Rails app, but the desired version may differ depending on the application being developed, the installation procedures may vary based on the developer's OS, and I generally want to avoid cluttering the local machine environment.
• I prepared a Docker container with ImageMagick installed and configured it to be used from the MiniMagick gem.

Implementation#

Directory Structure#

In addition to the usual Rails app structure, I added the script dev/start to run the application and docker-compose.yml, along with a configuration file config/initializer/minimagick.rb to allow MiniMagick to utilize the Docker container.

.
├── app
︙
└── config
│  └── initializers
│      └── minimagick.rb
├── dev
│   └── start
├── docker-compose.yml
└── tmp
    └── images

config/initializers/minimagick.rb#

MiniMagick executes the system's ImageMagick command internally, and there is an option to add a prefix to that command. By specifying docker exec <container_name> here, MiniMagick can use ImageMagick from the Docker container. https://github.com/minimagick/minimagick/blob/d484786f35e91f107836d3c86aca61d50a35820b/lib/mini_magick/tool.rb#L126

require "mini_magick"

if Rails.env.development?
  ::MiniMagick.configure { |config|
    config.cli = :imagemagick
    config.cli_prefix = %w[docker exec imagemagick]
  }
end

docker-compose.yml#

This configuration sets up a container for ImageMagick alongside other middleware like MySQL. This container is kept running, and commands are executed like docker exec imagemagick <ImageMagick command>. I used a Docker image for ImageMagick that I found by searching on Docker Hub: https://hub.docker.com/r/v4tech/imagemagick.

MiniMagick uses Dir.tmpdir for the location of the image files it handles, which means it utilizes the directory specified by the environment variable TMPDIR on Linux. Therefore, it is necessary to mount this directory from the host machine to the imagemagick container, which is specified in the volumes.

version: '3.7'
services:
  db:
    image: mysql:5.7
    environment:
      MYSQL_ROOT_PASSWORD: hoge
      MYSQL_USER: hoge
    ports:
      - "3307:3306"
  imagemagick:
    container_name: imagemagick
    image: v4tech/imagemagick
    volumes:
      - $TMPDIR:$TMPDIR
    tty: true
    command: "/bin/ash"

dev/start#

This script starts the container group and the Rails app while specifying the path for the environment variable TMPDIR.

#!/bin/sh -eu

SCRIPT_DIR=$PWD/$(dirname $0)
ROOT_DIR=$SCRIPT_DIR/..
TMPDIR="$ROOT_DIR/tmp/images" # Explicitly specify the TMPDIR path to share with the imagemagick container

docker-compose up -d
bin/rails server

With this implementation, MiniMagick will utilize ImageMagick inside the Docker container, allowing you to avoid installing ImageMagick on your local machine ☺️.

Note that this is specifically for development environments where you want to run the Rails app on the host machine. In production environments where the Rails app is executed as a Docker container, you can simply install ImageMagick in that container.