Docker Compact Manual (4: Create a custom image)

At first

This article "Docker Compact Manual" consists of 4 items. Other articles are here.

Purpose of this article

If you want to create your own app, you need a customized container. It included not only the image from Docker Hub but also the data I wanted to include You have to create a custom Docker image. This time we will focus on how to create a custom Docker image.

Dockerfile best practices

Things to check before creating a custom image Docker officially publishes Dockerfile best practices. If you don't follow this best practice to some extent It may be difficult to understand, difficult to use, unmaintainable, or buggy. It is recommended to create it based on this content.

Best practices for writing Dockerfiles

Those who thought it was English: raised_hand: There is a site translated by the Japanese localization project. : relaxed: Please access from the following site.

Dockerfile best practices

One thing to note is that you should check the update history. If the update history is old, you should also check the official website.

Create a custom image from a Dockerfile

There is also a method of creating a container image from a container (docker commit), It is common to store the Dockerfile and the data to be included in the image in the same working directory.

Instructions that can be specified in the Dockerfile

Note In the Dockerfile, the instructions are executed line by line in order from the beginning. In other words, if the description order is changed, an error will occur or an unintended container will be created.

Easy to get hooked on part 1

At what point is RUN, CMD, ENTRYPOINT running? ?? I made a mistake too. I think this should be remembered in the figure.

--RUN is the stage of image creation --CMD, ENTRYPOINT is the stage of container startup

Easy to get hooked on part 2

Proper use of CMD and ENTRY POINT How should I use them properly? ??

** Conclusion ** Use CMD in most cases. When using CMD, if the user explicitly writes a command, that will take precedence. For example, if a bug occurs in a custom container, if it is specified by ENTRYPOINT It cannot be changed because the forced command is executed. That bug will continue to occur. On the other hand, CMD allows the user to choose to leave it to the default command or execute an arbitrary command.

Sample example

Custom samples in Rails are also available. You can see that it is different from the instruction in the official image of Docker Hub.

Docker Hub Official Rails

FROM ruby:2.3

RUN apt-get update \
    && apt-get install -y --no-install-recommends \
        postgresql-client \
    && rm -rf /var/lib/apt/lists/*

WORKDIR /usr/src/app
COPY Gemfile* ./
RUN bundle install
COPY . .

EXPOSE 3000
CMD ["rails", "server", "-b", "0.0.0.0"]

Quickstart: Compose and Rails

FROM ruby:2.5
RUN apt-get update -qq && apt-get install -y nodejs postgresql-client
RUN mkdir /myapp
WORKDIR /myapp
COPY Gemfile /myapp/Gemfile
COPY Gemfile.lock /myapp/Gemfile.lock
RUN bundle install
COPY . /myapp

# Add a script to be executed every time the container starts.
COPY entrypoint.sh /usr/bin/
RUN chmod +x /usr/bin/entrypoint.sh
ENTRYPOINT ["entrypoint.sh"]
EXPOSE 3000

# Start the main process.
CMD ["rails", "server", "-b", "0.0.0.0"]

Even so, the Ruby version of Docker Hub is 2.3, and Quickstart is also 2.5. Too old ❗️❗️ I would like it to be version 2.6 or later.

So, to build a modern Rails container environment, customize the contents of Quickstart further. I think you should do it.

** Promotion ** Those who wondered if it would be easier to build a modern development environment with Rails Please visit this site I made.

Manual for building a stable development environment for "Rails6" with "Docker-compose"

Finally

I posted Docker in 4 copies. Thank you very much for those who have been with us until the end. I would appreciate it if you could use it as a reference ** LGTM </ font> **.