Difference between 'image' and 'build' within docker compose

Question

Please help me understand the difference between 'image' and 'build' within docker compose

Appoint

Answer 1

• image means docker compose will run a container based on that image
• build means docker compose will first build an image based on the Dockerfile found in the path associated with build (and then run a container based on that image).

PR 2458 was eventually merged to allow both (and use image as the image name when building, if it exists).

therobyouknow mentions in the comments:

dockerfile: as a sub-statement beneath build: can be used to specify the filename/path of the Dockerfile.

version: '3'
services:
  webapp:
    build:
      context: ./dir
      dockerfile: Dockerfile-alternate
      args:
        buildno: 1

Err

dockerfile: as a sub-statement beneath build: can be used to specify the filename/path of the Dockerfile - docs.docker.com/compose/compose-file/#build – Michalmichalak

@Michalmichalak Thank you. I have included your comment in the answer for more visibility. – Err

+1 thanks @VonC. I should also say that I believe that if the dockerfile: statement optionally isn't present, then docker-compose will look for the file named Dockerfile in the directory that docker-compose was launched from (i.e. the same folder within where the docker-compose.yml file resides). If there is a (relative) path specified directly after the build: statement (which also means no sub-statements), then docker-compose will look in there for the Dockerfile. The other way to specify a directory is by using the context: sub-statement with the path (as your example shows). – Michalmichalak

These options for specifying the Dockerfile are explained in the documentation, here: docs.docker.com/compose/compose-file/#build (thank you for reading my comments and feeding back.) +1 upvoted your answer and the original question. – Michalmichalak

What happens if we use both image and build? is there a priority between them or always the second one gets used? – Sharecropper

@AminBashiri You could not use it within the same service:. But you can use them both in the same services: Dockerfile, with depends_on to make sure the build is done first. github.com/compose-spec/compose-spec/blob/master/… – Err

what I have seen is using docker-compose -f a.yml -f b.yml and for a particular service, it uses build in one file and image in the other. – Sharecropper

@AminBashiri OK. I didn't know it could work for the same service. – Err

Answer 2

build: expects dockerfile path as an argument, it will build an image first and then use the image to create a container.

image: expects existing image name as argument , it will launch container using this image.

Example:docker-compose.yaml

version: '3'
services:
  service1:
    build: .
    ports:
      - "5000:5000"
  service2:
    image: "redis:alpine"

service1 will build an image first based on Dockerfile of current path and run container based on this image.

service2 will download "redis:alpine" image from docker hub and run container on downloaded image.

Kra

Answer 3

Official documentation that goes in depth on the difference between build and image might be of some help here. https://docs.docker.com/compose/compose-file/build/ Long story short, build needs the path to a Dockerfile that it can use to build an image, but image goes to a repository to grab an existing image. Both can be included for a service, but

When a service definition includes both the image attribute and a build section, the Compose implementation can’t guarantee a pulled image is strictly equivalent to building the same image from source. Without any explicit user directives, the Compose implementation with Build support MUST first try to pull the image, then build from source if the image was not found on registry. The Compose implementation MAY offer options to customize this behaviour by user request.

Bite

In my case, docker-compose up would build the image, docker compose up would pull then build the image. I haven't found a way for "pull if exist, build if not". – Sussi