​
Writing Dockerfiles for your docker-compose.yml

When writing the docker-compose.yml file, you’ll notice that you must write a Dockerfile for your appplication so that you can reference it in your compose file. Just as you’d do when containerizing your app for any other circumstances, this Dockerfile should package your application within a container image so that the application starts when you docker run a container using that image.
If you’re not familiar with Docker, we highly recommend reading Docker’s official “Get Started” page.
A typical Node.js Express app, for example, would have a Dockerfile similar to the one below. 
FROM node:16-alpine

WORKDIR /usr/src/app

# Install your app's dependencies
COPY package*.json ./
RUN npm install

# Copy the application code itself
COPY . .

# This is the port to which you'll bind Express
EXPOSE 3001

# Start the server
CMD ["node", "server.js"]
Once you’ve created a Dockerfile for your app, reference it in your docker-compose.yml, as shown below. 
version: "3.8"
services:
    api:
        build: ../path_to_your_dockerfile_folder
        ports:
            - "3001:3001"
Once you do that, Ergomake Bot will add a comment containing a link to your application. By now, you should have a preview environment up and running for your application. That’s good, but most applications depend on multiple other pieces of software, like databases. To learn how to set them up, see Databases and other third-party applications.

​
When to write a Dockerfile

Usually, you’ll only have to write a Dockerfile for applications you own and for which there isn’t a built image yet. Take it as a rule-of-thumb that you should not write Dockerfile for applications such as databases, or message queues. For those applications, you should usually default to picking an image from Docker Hub, which is Docker’s official image repository. We recommend that users only write Dockerfiles when they can’t find the software they need packed into a Docker Hub image.
If your integrations already build a Docker image as part of your pull-request process, we recommend that you reuse that image so that preview environments come up more quickly.

​
Setting build arguments

There will be cases in which a Dockerfile needs to have access to a particular service’s address during build-time. One such example is when you must bundle a front-end app including the value for the back-end address to which the web app will send requests, as shown in the example Dockerfile below. 
FROM node:16-alpine

ARG REACT_APP_API_URL

ENV REACT_APP_API_URL $REACT_APP_API_URL

# A bunch of other commands [...]

RUN npm run build

EXPOSE 8080
CMD ["npm", "run", "start-docker"]
In this example, we use the REACT_APP_API_URL build argument to set the environment variable REACT_APP_API_URL. This variable will be used during npm run build because it will go into the bundle so that the app knows what’s the backend to which it should send requests. To provide a build argument containing service’s public address, you’ll use the dev.ergomake.preview.replace-arg.YOUR_ENV_VAR label. Within that var, you can interpolate the address we’ll generate for a service, as shown in the example below. 
version: '3.8'
services:
   web:
     build:
       context: ./frontend
     ports:
       - "8080:8080"
     labels:
       # This label will cause Ergomake to set, during build time,
       # REACT_APP_API_URL to the address of the API service (https),
       # followed by "/prefix"
       dev.ergomake.env.replace-arg.REACT_APP_API_URL: 'https://{{ services.api.url }}/api'

  api:
    build: ./backend
    ports:
      - '3001:3001'