Docker compose and external images multi-stage builds

Question

I use Docker multi-stage build, specifically:

Use an external image as a “stage”

When using multi-stage builds, you are not limited to copying from stages you created earlier in your Dockerfile. You can use the COPY --from instruction to copy from a separate image, either using the local image name, a tag available locally or on a Docker registry, or a tag ID. The Docker client pulls the image if necessary and copies the artifact from there. The syntax is:

In my case I have three Docker files.

One Dockerfile simply defines an image, which I use as a build stage to share among two other Dockerfiles:

FROM ubuntu:bionic

## do some heavy computation that will result in some files that I need to share in 2 Dockerfiles

and suppose that I build the above Dockerfile giving:

docker build -t my-shared-build -f path/to/shared/Dockerfile .

So now I have a my-shared-build image that I share between two containers, their dockerfiles look like:

FROM ubuntu:bionic

# do something

COPY --from=my-shared-build some/big/folder /some/big/folder

CMD ["/my-process-one"]

FROM ubuntu:bionic

# do something

COPY --from=my-shared-build some/big/folder /some/big/folder

CMD ["/my-process-two"]

And I can build and run them.

So to recap, what I currently do is:

1) Build the shared image 2) Build the process one image 3) Build the process two image

now I can just run the "process one" and "process two" containers.

Problem

Now I would like to use Docker Compose to automate the execution of "process one" and "process two".

So the question is: how can I specify in the Docker Compose that I need to first build the shared image and then build the other images (and then run their containers) ?

Answer 1

I think you should be able to do that just fine.

docker-compose:

version: '3'
services:
    my-shared-build:
        image: my-shared-build:latest
        build: my-shared-build

    my-process-one:
        image: my-process-one:latest
        build: my-process-one
        depends_on:
            - my-shared-build

    my-process-two:
        image: my-process-two:latest
        build: my-process-two
        depends_on:
            - my-shared-build
            - my-process-one

Assuming your Dockerfiles are in subdirectories my-shared-build, my-process-one, my-process-two this should build all 3 images (in order)