What and Why Behind Multistage Builds
In a Dockerfile, each statement adds up a new layer to the image. It could be counterproductive if you also build your application package when building your Docker image. This could increase the size of our docker images substantially. That’s when we need to leverage multi-stage builds feature.
Simply put, multistage builds are useful when we want to clean up and reduce the image size without keeping unwanted artifacts lying around in our image.
Let’s understand this through an example.
A Concrete Example
I have created a sample React app and 2 Dockerfiles to demonstrate this, namely,
Typical Dockerfile without stages
FROM node:13.13.0-alpine WORKDIR /app COPY package.json ./ RUN npm install --silent RUN npm install email@example.com -g --silent COPY . ./ CMD ["npm", "start"]
Let’s build and run this Docker image:
docker build -t multi-stage-demo:dev -f Dev.Dockerfile .
In the lightweight version, we use the same base image to build the application, but we are using
nginx base image to run our application since we already have our application built.
# Stage 1 - the build process FROM node:13.13.0-alpine as build-deps WORKDIR /app COPY package.json ./ RUN npm install --silent RUN npm install firstname.lastname@example.org -g --silent COPY . ./ RUN npm run build # Stage 2 - the deploy process FROM nginx COPY --from=build-deps /app/build /usr/share/nginx/html EXPOSE 80 CMD ["nginx", "-g", "daemon off;"]
Here, you’d notice that we have two
FROM directives. The first one will be used for the base image for building the application and the second one for the final image. The built binaries are copied over from the first image over to the second and we expose port 80 to the host.
Now, let’s give this a go and build it:
docker build -t multi-stage-demo:prod -f Prod.Dockerfile .
Time to check our image sizes
docker images | grep multi-stage-demo
If we compare
prod image, that’s a ~73% reduction! 😍This can also benefit you in reduced deployment times.
The Github repo with code is available here
Where to From Here?
There are some nice examples and best practices defined in the official Docker documentation.
For example, if your build contains several layers, you can order them from the less frequently changed (to ensure the build cache is reusable) to the more frequently changed:
- Install tools you need to build your application
- Install or update library dependencies
- Generate your application