How does Docker buildx handle concurrency? Specifically in a (Rush) monorepo?

Question

We use rush for our typescript monorepo. One of the things you can do with rush is defined custom cli commands. We have one that builds a docker image for every project in our monorepo. In our CI pipeline, we run this custom rush command to build images and push them to our registry. We use Github Actions.

For reasons I won't go into, our current setup involves running the rush install and rush deploy commands outside of the docker image, and then we copy into the dockerfile. I want to fix that and keep everything within docker.

Rush will (effectively) run a parallel build for each app. But how does the Docker engine handle multiple invocations of docker buildx build ... at the same time?

If I have a multi-stage docker image which is pretty static (uses no build args) to move the source code and install dependencies, then the next stage uses --build-args to customize that particular app (ie rush build --to=<project>), will the docker engine know to build those first layers before allowing the parallel processes to continue?

I'll attach a snippet of what I've come up with, but even running commands locally I can't really tell what the behavior is.

### BASE
FROM node:18-alpine as base

WORKDIR /platform

RUN npm install -g @microsoft/[email protected]

COPY rush.json .

### CODE -- source code, etc. Installed
FROM base AS code

WORKDIR /platform

# Rush dependencies
COPY common/config ./common/config
COPY common/scripts ./common/scripts
# Source code
COPY apps/ ./apps
COPY libs/ ./libs

RUN rush install

### DEV - Run the app
FROM code AS dev

WORKDIR /platform/apps/${APP_NAME}

ENTRYPOINT npm run serve

### BUILD - build the source (for prod)
FROM code AS build

WORKDIR /platform

ARG APP_NAME
ARG DIST_FOLDER=dist
ARG DEPLOY_FOLDER=deploy
ARG JS_ENTRYPOINT=main.js
ARG NODE_ENV=production
# Used for 'pkg' to target a specific arch/platform. 
# Adjust if building locally, ie `TARGET_ARCHITECTURE=arm64` and `TARGET_PLATFORM=macos`
ARG TARGET_ARCHITECTURE=x64
ARG TARGET_PLATFORM=alpine

ENV JS_ENTRYPOINT=${JS_ENTRYPOINT}
ENV NODE_ENV=${NODE_ENV}
ENV APP_NAME=${APP_NAME}
ENV DIST_FOLDER=${DIST_FOLDER}
ENV DEPLOY_FOLDER=${DEPLOY_FOLDER}

RUN rush build --to=@org/${APP_NAME}

WORKDIR /platform/apps/${APP_NAME}

# pkg the app for tiny binaries/image size
RUN npm i -g pkg

# build for target 'node18-alpine-x64' by default.
RUN pkg ${DIST_FOLDER}/${JS_ENTRYPOINT} -o ${APP_NAME}_binary -t node18-${TARGET_PLATFORM}-${TARGET_ARCHITECTURE}

### PROD -- minimal prod build, copying the built binary into an alpine image
FROM alpine AS prod

WORKDIR /platform

ARG APP_NAME
ENV APP_NAME=${APP_NAME}

COPY --from=build /platform/apps/${APP_NAME}/${APP_NAME}_binary .

ENTRYPOINT ${APP_NAME}_binary