Parsing of key-value build args in docker-compose not working

[bilow-factual]

Expected behavior

Should be able to specify build args as key-value pairs, strings with assignment, or strings without assignment, as in: https://docs.docker.com/compose/compose-file/#args

Actual behavior

Error below when passing build args as key-value pairs. See Dockerfile + docker-compose.yml below for more information:

ERROR: The Compose file './docker-compose.yml' is invalid because: services.dev.build.args contains {"goodbye": "bye"}, which is an invalid type, it should be a string

Information

Docker for Mac: version: 17.12.0-ce-mac55 (18467c0ae7afb7a736e304f991ccc1a61d67a4ab)
macOS: version 10.13.3 (build: 17D102)
logs: /tmp/F873DA0F-6DAF-44C1-A4D4-5154F96751D5/20180304-185129.tar.gz
[OK]     vpnkit
[OK]     vmnetd
[OK]     dns
[OK]     driver.amd64-linux
[OK]     app
[OK]     virtualization VT-X
[OK]     moby
[OK]     system
[OK]     moby-syslog
[OK]     kubernetes
[OK]     env
[OK]     virtualization kern.hv_support
[OK]     moby-console
[OK]     osxfs
[OK]     logs
[OK]     docker-cli
[OK]     disk

Steps to reproduce the behavior

Here's my docker-compose.yml

version: '3'
services:
  dev:
    build:
      context: .
      args:
        - hello=hi
        - goodbye: bye

and my Dockerfile

FROM ubuntu
ARG hello
ARG goodbye
RUN echo $hello
RUN echo $goodbye

1. docker-compose up yields the error above, complaining about the type of the 'goodbye' build arg.

[thaJeztah]

The build args can be either provided as a list, or as a mapping; referring to the docs:

Then specify the arguments under the build key. You can pass either a mapping or a list:

build:
  context: .
  args:
    buildno: 1
    password: secret

build:
  context: .
  args:
    - buildno=1
    - password=secret

You can omit the value when specifying a build argument, in which case its value at build time is the value in the environment where Compose is running.

args:
  - buildno
  - password

So either specify (list of strings):

args
  - key=value
  - key2

Or (mapping)

args
  key: value

The reason it's failing in your case is because you're trying to use a "map" (key: value) inside a list (starting with - )

Change your compose file to:

version: '3'
services:
  dev:
    build:
      context: .
      args:
        hello: hi
        goodbye: bye

Or

version: '3'
services:
  dev:
    build:
      context: .
      args:
        - hello=hi
        - goodbye=bye

And it should work

I'm closing this issue, because this doesn't look like a bug, but feel free to continue the conversation 👍

[bilow-factual]

Ah, oops. Thank you.

[docker-robott]

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