Better caching multi-stage builds

[alexander-xyz]

Hi!
I want to know, how to better caching multi-stage builds.
For example, this is my dockerfile:

FROM php:7.2-fpm-alpine as php-image

# run some commands

FROM node:14-alpine AS node-image
RUN npm install -g grunt-cli npm@latest

FROM php-image as builder

COPY --from=node-image /usr/lib /usr/lib
COPY --from=node-image /usr/local/share /usr/local/share
COPY --from=node-image /usr/local/lib /usr/local/lib
COPY --from=node-image /usr/local/include /usr/local/include
COPY --from=node-image /usr/local/bin /usr/local/bin
COPY --from=node-image /opt /opt

RUN npm config -g set user root

Do I have to build and cache the intermediate stages with a --opt target= for multistage builds, for better caching?

buildctl build ...\
  --import-cache type=registry,ref=docker.io/username/image:php-image
  --export-cache type=registry,ref=docker.io/username/image:php-image,mode=max
  --opt target=php-image

buildctl build ...\
  --import-cache type=registry,ref=docker.io/username/image:node-image
  --export-cache type=registry,ref=docker.io/username/image:node-image,mode=max
  --opt target=node-image

buildctl build ...\
  --import-cache type=registry,ref=docker.io/username/image:php-image
  --import-cache type=registry,ref=docker.io/username/image:node-image
  --import-cache type=registry,ref=docker.io/username/image:builder
  --export-cache type=registry,ref=docker.io/username/image:builder,mode=max
  --opt target=builder

Or, I can build just need target, and all intermediate stages will be cached well?

buildctl build ...\
  --import-cache type=registry,ref=docker.io/username/image:builder
  --export-cache type=registry,ref=docker.io/username/image:builder,mode=max
  --opt target=builder

Thank you!

[tonistiigi]

The last command is fine.

In fact, if you export cache for only your php-image stage the layers are not currently exported because BuildKit atm does not export cache for an image if there is nothing using that image. This is because it is expected that pulling an image again and pulling the cache are similar operations in terms of performance.

You may also want to look into COPY --link https://www.docker.com/blog/image-rebase-and-improved-remote-cache-support-in-new-buildkit/ .