context or workdir for docker-compose

I'm learning docker

I need to specify the working directory for a docker image, I think that'll be something like this:

version: '2'
services:
  test:
    build:
      context: ./dir

Now I want to make the image python:onbuild to run on the ./dir, but I dont want to create any Dockerfile inside the ./dir.

The docker-compose manual says nothing about that.

Is it possible? How to do that?

I think you're looking for working_dir. Search for "working_dir" in the docker-compose reference.

You can specify the working directory as follows.

version: '2'
services:
  test:
    build:
      context: ./dir
    working_dir: /dir

Possibly not exactly what you were looking for, but you can specify the "context" for the docker build to be a different directory to where the actual Dockerfile lives.

build:
  context: ./folder/containing/files
  dockerfile: path/to/dockerfile/relative/to/context/Dockerfile

Any ADD/COPY commands in your Dockerfile then act as if they are relative to the context regardless of where the Dockerfile actually is.

The build configuration in Docker Compose just ends up in a call to docker build, so you need to have a Dockerfile to use that workflow.

As the docs for python:onbuild say, you can start with a minimal Dockerfile that just contains FROM python:onbuild. But as they also say, :onbuild isn't a great option, you'll have much more control building your own Dockerfile FROM python.