Developing With Docker • Christopher Pietrzykowski

Just in case you’ve been living under a rock for the last 5 years or so, here’s a quick primer. Docker is a virtualisation/containerisation platform. It uses a description file (that you create) to build and apply layers to a base image. This allows containers to remain small, and only the layer needs to be updated when there’s a change. The image you build is a portable package, your application and complete dependencies.

This saves you from building entire VMs, maintaining the core system on top of just your application.

I’ve been using Docker for web/server development for a little over a year now. It is an indispensable tool for discrete project setup. I never have to install specific dependencies, or pieces of a stack to my host. In development, you may run into a few host->container mysteries – notoriously, fs updates. Most packages have a slower poll mode to make up for this. Inconvenience for sure, hopefully that’ll be a relic soon enough.

Quick nodemon example setup

Create a Dockerfile – this is your application
- pick a suitable base
```
FROM node
WORKDIR /var/app
EXPOSE 3000
ENTRYPOINT ["/var/app/dev-entrypoint.sh"]
```
- here’s an example development entrypoint, this makes sure that dependencies are installed after the container is started.
```
#!/bin/sh
yarn install
yarn dev
```

Create a docker-compose.yml – compose connects containers

typically I don’t need to do anything special to base database images
- initial migrations, schema/config, etc.
```
FROM postgres
COPY ./docker-entrypoint-initdb.d/* /docker-entrypoint-initdb.d
```

add a volumes section to persist dev/test data, and to connect your application code (for nodemon)

version: '3'

services:
    server:
        build: ./server
        env_file: ./server/dev.env
        volumes:
        - ./server:/var/app
        ports:
        - "3000:3000"
        depends_on:
        - db

    db:
        build: ./db
        ports:
        - "5432:5432"
        volumes:
        - pgdata:/var/lib/postgresql/data
        environment:
        POSTGRES_PASSWORD: secret

volumes:
    pgdata:

compose sets up hostnames (by service name) for containers that need to connect between each other.
- e.g., to connect to the db from server use db:5432

> docker-compose up

Simple, easy to extend. A practical addition might be a container for front-end development.