I've been working on a fullstack project that's end-to-end JavaScript, and have a Dockerfile that simplifies to this
FROM node:18-slim AS backend-builder
WORKDIR /backend-staging
COPY ./backend/package.* .
RUN npm install
# bring in other files
FROM node:18-slim AS frontend-builder
WORKDIR /frontend-staging
COPY ./frontend/package.* .
RUN npm install
# bring in other files, run webpack, etc
FROM node:18-slim AS final
WORKDIR /core
COPY ./package.* # root file w/ some non-webpack'd externals and scripts to access after the contianer builds
RUN npm install
COPY --from=backend-builder /backend-staging/build/. .
COPY --from-frontend-builder /frontend-staging/build ./webapp
# setup some postinstall things, set start command, fin
When I run docker build, after pulling the node:18-slim image, I see that buildkit parallelization takes place across my stages, with all 3 npm install commands showing up in the output at the same time. However, it seems that the actual installation goes one stage at a time, with the output first appearing from the final stage and running to completion, then the backend, then the frontend, before the copying and whatnot resumes. It almost seems like there's a mutex on access to the actual npm program, so while the command can be parallelized, the execution is first-come-first-serve single threaded.
I was reading this question, but I seem to have a different problem, as the build stages are definitely being parallelized; their timers all start simultaneously. It's the commands within the build stages that are serialized across stages, for seemingly no reason.
I'm accessing the Docker engine/daemon via Docker Desktop integration w/ WSL2, with buildkit enabled through my Docker Desktop config. The exact command I'm running to build is docker build . -t 'some-image-tag'.
