In this lesson, you will learn how to configure Docker to run independent build stages concurrently to significantly slash your image creation time.
The Concept of Parallelism
When you write a multi-stage Dockerfile, Docker executes each FROM instruction sequentially by default. However, many applications require separate build processes that don’t rely on one another—for example, building a frontend static asset bundle and compiling a backend Go binary.
If your frontend build takes three minutes and your backend build takes three minutes, running them sequentially wastes six minutes. By structuring your stages correctly, Docker’s BuildKit engine can identify these independent branches and execute them simultaneously, reducing that total time to roughly three minutes.
How it Works
To enable parallel execution, you define independent stages that do not share a dependency chain. Docker effectively builds a dependency graph; if Stage A and Stage B both start from a base image and do not COPY from each other, they are candidates for parallel processing.
Consider this structure:
# Stage 1: Build the Frontend
FROM node:18 AS frontend-builder
COPY ./ui .
RUN npm install && npm run build
# Stage 2: Build the Backend
FROM golang:1.21 AS backend-builder
COPY ./api .
RUN go build -o server .
# Stage 3: Final Assembly
FROM alpine:latest
COPY --from=frontend-builder /app/dist /www
COPY --from=backend-builder /app/server /bin/server
In the example above, because frontend-builder and backend-builder are independent of each other, BuildKit will spawn two separate workers to process them at the same time. The final stage will wait until both are complete before pulling the artifacts into your production-ready image.
This approach is particularly powerful in CI/CD pipelines where build runners are often constrained by time. By ensuring your Dockerfile branches are independent rather than chained linearly, you make the most of your hardware resources and shorten your feedback loop.
Key Takeaway: By structuring your multi-stage Dockerfile with independent branches, BuildKit automatically executes non-dependent stages in parallel, drastically reducing your total build time.