5.2.1.2 BuildKit Parallel Builds

BuildKit parallel builds refers specifically to BuildKit's ability to execute multiple independent stages, or independent steps within the same build, concurrently rather than strictly in the linear order they happen to appear in the Dockerfile.

Structuring a Dockerfile to Enable Parallelism

Parallelism opportunities arise specifically when a Dockerfile defines stages with no dependency relationship between them — BuildKit can only parallelize what is genuinely independent, so structuring stages deliberately with this in mind is what unlocks the benefit.

FROM python:3.12 AS test
RUN pip install -r requirements-test.txt
RUN pytest

FROM python:3.12-slim AS runtime
COPY . /app
CMD ["python", "/app/main.py"]

The test stage and the runtime stage here share no dependency, allowing BuildKit to potentially run them concurrently rather than strictly one after the other.

Observing Parallel Execution

Build output with sufficient detail can reveal multiple stages actively executing at overlapping points in time, rather than one completing entirely before the next begins.

docker build --progress=plain -t myapp .

Why Not Every Build Benefits Equally

A Dockerfile whose stages form a long, strictly dependent chain — each one needing the previous stage's output before it can begin — offers little or no opportunity for parallelism, regardless of how capable the underlying build engine is, since the dependency relationship itself is what limits concurrent execution.

FROM golang:1.22 AS build
RUN go build -o app .

FROM alpine
COPY --from=build /app /app

This structure has no independent stages to parallelize, since the second stage strictly depends on the first.

Designing for Parallelism Deliberately

Recognizing genuinely independent pieces of a build — running tests, building a frontend bundle, compiling a backend binary — and structuring them as separate, non-dependent stages specifically to take advantage of BuildKit's parallel execution is a deliberate design choice, not something that happens automatically regardless of structure.

Why Parallel Builds Matter

For projects with genuinely independent build concerns, deliberately structuring a Dockerfile to expose that independence to BuildKit can produce a meaningfully faster build than an equivalent, more strictly sequential structure would allow.