dockerfile_inline limitations

[ToshY]

Is this a docs issue?

• My issue is about the documentation content or website

Type of issue

Information is incorrect

Description

The dockerfile_inline documentation shows a basic example on how to use multi-line syntax to pass the contents of a Dockerfile, but it seems that it cannot replace a Dockerfile out of the box, as I noticed instructions like ARG do not work.

I've noticed that it was already stated in compose-spec/compose-spec#298 (comment) that the use-case for dockerfile_inline would be for simple commands, so I think it doesn't hurt to explicitly mention this in the documentation.

Location

https://docs.docker.com/compose/compose-file/build/#dockerfile_inline

Suggestion

Add a note to the dockerfile_inline section explicitly stating that it's use-case is for simple instructions only and that it cannot fully replace using a separate Dockerfile.

[denzuko]

In an ecosystem where buildpacks, helm charts, and the like. I would argue for the opposite and allow it to fully replace a Dockerfile.

[ndeloof]

ARG is well supported, but - like when using a plain Dockerfile - you need to inform Compose about build args and value to be set:

$  cat compose.yaml 
services:
  test:
    build:
      dockerfile_inline: |
        FROM alpine
        ARG FOO
        RUN echo $FOO > /foo
      args:
        - FOO
$ FOO=BAR docker compose build
[+] Building 0.1s (6/6) FINISHED                                                                                                docker:desktop-linux
 => [test internal] load .dockerignore                                                                                                          0.0s
 => => transferring context: 2B                                                                                                                 0.0s
 => [test internal] load build definition from Dockerfile                                                                                       0.0s
 => => transferring dockerfile: 114B                                                                                                            0.0s
 => [test internal] load metadata for docker.io/library/alpine:latest                                                                           0.0s
 => CACHED [test 1/2] FROM docker.io/library/alpine                                                                                             0.0s
 => [test 2/2] RUN echo BAR > /foo                                                                                                              0.1s
...

[ndeloof]

Closing this issue as "work as documented"

[ToshY]

@ndeloof

ARG is well supported, but - like when using a plain Dockerfile - you need to inform Compose about build args and value to be set

Here's a simple example which uses ARG before FROM and includes a default value.

Dockerfile

ARG PHP_VERSION=8.2
FROM php:${PHP_VERSION}

This does not work with dockerfile_inline.

docker-compose.yml

version: '3.9'

services:
  php:
    build:
      context: .
      dockerfile_inline: |
        ARG PHP_VERSION=8.2
        FROM php:${PHP_VERSION}

$ docker compose up -d
WARN[0000] The "PHP_VERSION" variable is not set. Defaulting to a blank string.
[+] Building 0.0s (2/2) FINISHED                                                                                                                                             docker:default
 => [php internal] load build definition from Dockerfile                                                                                                                               0.0s
 => => transferring dockerfile: 68B                                                                                                                                                    0.0s
 => [php internal] load .dockerignore                                                                                                                                                  0.0s
 => => transferring context: 2B                                                                                                                                                        0.0s
failed to solve: failed to parse stage name "php:": invalid reference format

[ndeloof]

in your example, FROM php:${PHP_VERSION} is interpolated by docker compose during parsing, as demonstrated by the warning message about unset variable. You need to double the dollar sign to prevent this:

      dockerfile_inline: |
        ARG PHP_VERSION=8.2
        FROM php:$${PHP_VERSION}

[ToshY]

@ndeloof Thanks for the explanation!

[docker-robot]

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