Tips for writting docker files

Docker has revolutionized how developers build and deploy applications. Docker has support for different programming languages and runs natively on linux, as opposed to virtual machines that mimics an entire operational system, docker containers run on linux namespaces, removing the overhead that virtual machines have, for example, the boot time. The virtual machine needs time to boot, while docker is a service that starts in the operational system.

As opposed to the official best practices [1] on writting docker files, the goal here is to share tips on approaches in how to write the docker files, this is not a beginners guide in how docker works or how to use it.

Docker images and services

In this section, the focus is to go through the tips around building a docker image and the docker compose services.

1. Make the basic setup with standard image

Starting to build docker images requires steps and previous knowledge on the docker platform and ad least to understand a few commands such as RUN, COPY and FROM. Based on those commands, the generated image can be big or small, it depends.

Docker hub offers ready to use images without the need to build one, and they are classified as (from the biggest to the smallest): standard image, slim and alphine.

Dockerfile with standard image:

FROM node:12 # <--- standard image, and also the bigger compared to the next two version

WORKDIR /var/www/app

COPY package*.json ./

COPY . .

RUN npm install && npm run build

EXPOSE 5000

CMD npm run serve

Dockerfile with slim image:

FROM node:12-slim # <--- slim image, smaller, but also has less dependencies installed by default

WORKDIR /var/www/app

COPY package*.json ./

COPY . .

RUN npm install && npm run build

EXPOSE 5000

CMD npm run serve

Dockerfile with alphine image:

FROM node:12-slim # <--- slim image, the smallest, but also it has some drawbacks such as missing needed dependencies by the code

WORKDIR /var/www/app

COPY package*.json ./

COPY . .

RUN npm install && npm run build

EXPOSE 5000

CMD npm run serve

Usually the setup using the basic image is faster as it comes with almost everything to run the program. on the other side though, the alphine version has almost nothing to run the program, it has just the core, nothing else. Which in many cases will make the program to not run, depending on the dependencies.

2. The root user

The root user is the default user in which the container runs, which makes easier the process to set up permissions to access files or to setup configurations. Usually this is a bad practice to, the container should not run with the root user due security issues [2].

Though for the process to set up the docker image this can be a bit harder, given the fact that setting up a different user with less permission can difficult the image setup.

If no user is given (as for example, the last 3 dockerfiles shown in the previous section), docker will build it using root, which of course has security issues. To fix this issue docker offers the flag USER.

FROM node:12-slim

USER node # <!--- specify the user for docker to build and run the image

WORKDIR /var/www/app

COPY package*.json ./

COPY . .

RUN npm install && npm run build

EXPOSE 5000

CMD npm run serve

This tip relies on the same approach as the previous one, first, make it work with the root user, then start to trick around permissions with a specific user.

3. Separate concerns, avoid building different services into one image

As a best practice the recommended way to build containers is: one container equals to one process. Which can avoid problems when it comes to managing them.

4. setup docker file first, and then move to docker compose (if needed)

Usually, docker compose is the next step when building services to use with docker, though developers tend to skip the first step which is to understand how the image works and then move on to compose.

5. Networking and sharing hosts

Docker creates its own network interface, which in turn containers communicate between each other. Therefore, there are scenarios in which this behavior is not desired. For example, a database. As the database holds state (the data) usually it is used an external provider (RDS, mongodb atlas etc).

By default the container can’t access external ports, which in turn will block the database connection. There are two possible options for that, using a network flag or using the add-host flag.

# using --network flag
docker run --rm --network=host nginx

There is a side effect using the network flag, which will ignore the docker network created automatically by docker and the container will run as if it were in the host. Impacting the port that the application run and therefore prevents the possibility of blue-green deployments [3], which requires two instances of the same app running, each on its specific port.

The add-host gives the flexibility needed to overcome the port issue. The flag maps a specific host to a IP, the following example maps the localhost to be the host.

# using --add-host flag
docker run --rm --add-host=localhost:192.168.1.102 nginx

Docker compose

This section focus on the docker compose only.

6. Different docker compose files for different environments

Docker compose files are used to compose the container orchestration, therefore sometimes it is needed to use different behavior based on the environment that the application is one. For example, in development mode, the database container might be needed, but in production it might not be the case.

For that, it is possible to create different docker compose files for each environment. For example, for development, staging and production we might have:

  • development: docker-compose-dev.yml
  • staging and production: docker-compose-deploy.yml

It is also possible to share code among each docker file, which might make sense to create a docker-compose.yml as the base for the two files previously mentioned.

References

  1. [1]DockerHub, “Best practices for writing Dockerfiles” [Online]. Available at: https://docs.docker.com/develop/develop-images/dockerfile_best-practices. [Accessed: 23-May-2020]
  2. [2]B. Fisher, “Docker and Node.js Best Practices from Bret Fisher at DockerCon,” 2019 [Online]. Available at: http://www.youtube.com/watch?v=Zgx0o8QjJk4. [Accessed: 23-May-2020]
  3. [3]Marcus, “How to do Zero Downtime Deployments of Docker Containers,” 2019 [Online]. Available at: https://coderbook.com/@marcus/how-to-do-zero-downtime-deployments-of-docker-containers. [Accessed: 2020-Jun-11AD]
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