Migrate existing Rails 6 apps to Docker environment


We were developing using rails in the local environment, but in consideration of future deployment, we decided to manage the environment in a container with Docker. It took a long time for the first time, so I will leave it as a memorandum. I hope it will be helpful for those who need to build a similar environment.

Goals for building an environment

Built environment

Container configuration



Only the parts changed this time are listed. The root directory is the root directory of the rails app.

├── config
│   ├── database.yml    #update
│   └── webpacker.yml   #update
├── docker
│   └── rails
│       └── Dockerfile  #Create New
├── docker-compose.yml  #Create New
├── .env                #Create New
└── Gemfile             #update



#Alpine is used for weight reduction.
FROM ruby:2.7.2-alpine3.12

ENV TZ="Asia/Tokyo" \
    LANG="C.UTF-8" \
    APP_ROOT="/app" \


#Introduction of ENTRY KIT
RUN wget https://github.com/progrium/entrykit/releases/download/v${ENTRYKIT_VERSION}/entrykit_${ENTRYKIT_VERSION}_Linux_x86_64.tgz \
    && tar -xvzf entrykit_${ENTRYKIT_VERSION}_Linux_x86_64.tgz \
    && rm entrykit_${ENTRYKIT_VERSION}_Linux_x86_64.tgz \
    && mv entrykit /bin/entrykit \
    && chmod +x /bin/entrykit \
    && entrykit --symlink

RUN apk update \
&&  apk add --no-cache \
    alpine-sdk \
    bash \
    build-base \
	mysql-client \
    mysql-dev \
    nodejs \
    tzdata \


  "prehook", "bundle install -j4 --quiet", "--", \
  "prehook", "yarn install --check-files --ignore-optional", "--"]

Image weight reduction

Alpine is used to reduce the weight of the image for the efficiency of future work. The capacity has been reduced from about 1.6GB to about 400MB.

By using a multi-stage build and installing nodejs separately, it seems that the image can be further reduced in weight. I was able to reduce the weight to some extent, so this time I decided to stop.

Introduction of ENTRY KIT

I am using ENTRY KIT to execute yarn install and bundle install when the container is started. In the articles I examined, most of them were executed in the Dockerfile, but due to the following problems, I forgot this time.

Therefore, it is configured to solve the above problem by executing it when the container is started and making the module persistent.



version: '3'

    image: mysql:8.0.22
    command: mysqld --character-set-server=utf8 --collation-server=utf8_unicode_ci
      - '3306:3306'
      - db_data:/var/lib/mysql

  web: &app_base
      context: .
      dockerfile: ./docker/rails/Dockerfile
    command: bash -c "rm -f tmp/pids/server.pid && bundle exec rails s -p 3000 -b"
      - db
      - "3000:3000"
      - .:/app
      - gem_modules:/vendor/bundle
      - node_modules:/node_modules
    tty: true            #binding.For pry execution
    stdin_open: true     #binding.For pry execution
      WEBPACKER_DEV_SERVER_HOST: webpacker    #webpack_dev_Specify the container for server execution
      BUNDLE_APP_CONFIG: ./.bundle
      NODE_ENV: development
      RAILS_ENV: development

  webpacker:             #webpack_dev_container for server execution
    <<: *app_base
    command: bash -c "bundle exec bin/webpack-dev-server"
      - web
      - '3035:3035'
    tty: false           #binding.Change to false because pry is unnecessary
    stdin_open: false    #binding.Change to false because pry is unnecessary
      BUNDLE_APP_CONFIG: ./.bundle
      NODE_ENV: development
      RAILS_ENV: development




 default: &default
-  adapter: sqlite3
+  adapter: mysql2
+  username: app
+  password: password
+  host: db    #Specify the service name

   <<: *default
-  database: db/development.sqlite3
+  database: mysql_development



 default: &default
-    host: localhost
+    host: webpack    #Specify the service name



-gem 'sqlite3', '~> 1.4'
+gem 'mysql2', '0.5.3'




Questions / things that could not be resolved

First container startup error (stack Error: getaddrinfo EAI_AGAIN nodejs.org)

An error may occur during yarn install when the container is started for the first time. As a result of investigation, it seems that the cause is that the name cannot be resolved by DNS.

I tried changing the DNS on the host side, which seems to be the solution, and specifying the DNS in docker-compose.yml, but it did not solve. The error did not occur at the second startup, and there was an article that the solution was to restart the container, so leave it as it is.

error /app/node_modules/node-sass: Command failed.
web_1        | Exit code: 1
web_1        | Command: node scripts/build.js
web_1        | Arguments: 
web_1        | Directory: /app/node_modules/node-sass
web_1        | Output:
web_1        | Building: /usr/bin/node /app/node_modules/node-gyp/bin/node-gyp.js rebuild --verbose --libsass_ext= --libsass_cflags= --libsass_ldflags= --libsass_library=
web_1        | gyp info it worked if it ends with ok
web_1        | gyp verb cli [
web_1        | gyp verb cli   '/usr/bin/node',
web_1        | gyp verb cli   '/app/node_modules/node-gyp/bin/node-gyp.js',
web_1        | gyp verb cli   'rebuild',
web_1        | gyp verb cli   '--verbose',
web_1        | gyp verb cli   '--libsass_ext=',
web_1        | gyp verb cli   '--libsass_cflags=',
web_1        | gyp verb cli   '--libsass_ldflags=',
web_1        | gyp verb cli   '--libsass_library='
web_1        | gyp verb cli ]
web_1        | gyp info using [email protected]
web_1        | gyp info using [email protected] | linux | x64
web_1        | gyp verb command rebuild []
web_1        | gyp verb command clean []
web_1        | gyp verb clean removing "build" directory
web_1        | gyp verb command configure []
web_1        | gyp verb check python checking for Python executable "python2" in the PATH
web_1        | gyp verb `which` succeeded python2 /usr/bin/python2
web_1        | gyp verb check python version `/usr/bin/python2 -c "import sys; print "2.7.18
web_1        | gyp verb check python version .%s.%s" % sys.version_info[:3];"` returned: %j
web_1        | gyp verb get node dir no --target version specified, falling back to host node version: 12.20.1
web_1        | gyp verb command install [ '12.20.1' ]
web_1        | gyp verb install input version string "12.20.1"
web_1        | gyp verb install installing version: 12.20.1
web_1        | gyp verb install --ensure was passed, so won't reinstall if already installed
web_1        | gyp verb install version not already installed, continuing with install 12.20.1
web_1        | gyp verb ensuring nodedir is created /root/.node-gyp/12.20.1
web_1        | gyp verb created nodedir /root/.node-gyp
web_1        | gyp http GET https://nodejs.org/download/release/v12.20.1/node-v12.20.1-headers.tar.gz
web_1        | gyp WARN install got an error, rolling back install
web_1        | gyp verb command remove [ '12.20.1' ]
web_1        | gyp verb remove using node-gyp dir: /root/.node-gyp
web_1        | gyp verb remove removing target version: 12.20.1
web_1        | gyp verb remove removing development files for version: 12.20.1
web_1        | gyp ERR! configure error 
web_1        | gyp ERR! stack Error: getaddrinfo EAI_AGAIN nodejs.org
web_1        | gyp ERR! stack     at GetAddrInfoReqWrap.onlookup [as oncomplete](dns.js:66:26)
web_1        | gyp ERR! System Linux 4.19.121-linuxkit
web_1        | gyp ERR! command "/usr/bin/node" "/app/node_modules/node-gyp/bin/node-gyp.js" "rebuild" "--verbose" "--libsass_ext=" "--libsass_cflags=" "--libsass_ldflags=" "--libsass_library="
web_1        | gyp ERR! cwd /app/node_modules/node-sass
web_1        | gyp ERR! node -v v12.20.1
web_1        | gyp ERR! node-gyp -v v3.8.0
web_1        | gyp ERR! not ok 

Image weight reduction in multi-stage builds

I will try it when further efficiency is needed, such as shortening the time.


There was an article about building a new Rails app with Docker, but I had a hard time because there weren't many articles about containerizing existing Rails apps. Since it was built once as a development environment, we will consider and work on how to divide and build the test environment and production environment.

In addition, I would like to work on the following contents, so I will write an article if possible.

Articles that I used as a reference

** ・ How to create a Rails on Docker environment that is easy to develop --Qiita ** ** ・ From Rails new until it works in production mode --Qiita ** ** ・ Building Docker environment with Rails 6.0 x MySQL8 (Alpine base) --Qiita ** ** · More efficient with Docker Compose volumes-Qiita ** ** ・ [Dockerfile full explanation] Create the simplest Docker image for Rails production environment --Qiita ** ** ・ [Docker] I want to know about Rails development! Talk about improving work efficiency by making gem persistent | Enjoy IT Life ** ** ・ Points to keep in mind when building an environment with docker-compose --Qiita ** ** ・ Build Rails + webpacker on Docker environment with docker-compose --RoadMovie **

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