Using docker --squash in docker-compose when building images

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Is there a way to use the --squash option in docker-compose when building new docker images? Right now they have implemented --squash in docker as of 6 months ago, but I have not seen any docs about how to use this in docker-compose.yml.

Is there a work around here? (I see an open issue filed requesting this feature)

dockerfile docker-compose docker
2 Answers

Instead of using --squash, you can use Docker multi-stage builds.

Here is a simple example for a Python app that uses the Django web framework. We want to separate out the testing dependencies into a different image, so that we do not deploy the testing dependencies to production. Additionally, we want to separate our automated documentation utilities from our test utilities.

Here is the Dockerfile:

# the AS keyword lets us name the image
FROM python:3.6.7 AS base
WORKDIR /app
RUN pip install django

# base is the image we have defined above
FROM base AS testing
RUN pip install pytest

# same base as above
FROM base AS documentation
RUN pip install sphinx

In order to use this file to build different images, we need the --target flag for docker build. The argument of --target should name the name of the image after the AS keyword in the Dockerfile.

Build the base image:

docker build --target base --tag base .

Build the testing image:

docker build --target testing --tag testing .

Build the documentation image:

docker build --target documentation --tag documentation .

This lets you build images that branch from the same base image, which can significantly reduce build-time for larger images.

You can also use multi-stage builds in Docker Compose. As of version 3.4 of docker-compose.yml, you can use the target keyword in your YAML.

Here is a docker-compose.yml file that references the Dockerfile above:

version: '3.4'

services:
    testing:
        build:
            context: .
            target: testing
    documentation:
        build:
            context: .
            target: documentation

If you run docker-compose build using this docker-compose.yml, it will build the testing and documentation images in the Dockerfile. As with any other docker-compose.yml, you can also add ports, environment variables, runtime commands, and so on.

user_79904090

You can achieve squash result with trick like

FROM oracle AS needs-squashing
ENV NEEDED_VAR some_value
COPY ./giant.zip ./somewhere/giant.zip
RUN echo "install giant in zip"
RUN rm ./somewhere/giant.zip

FROM scratch
COPY --from=needs-squashing / /
user_65157550
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