Build process failed: Unknown instruction: ARG

Question

I'm trying my first Docker Hub automated build using a Dockerfile I have successfully built locally. On Docker Hub, it fails with this output

Cloning into 'brx9syjpjlk9bnmymmynwpl'...
Warning: Permanently added the RSA host key for IP address '104.192.143.2' to the list of known hosts.
KernelVersion: 3.13.0-40-generic
Os: linux
BuildTime: Mon Oct 12 05:37:18 UTC 2015
ApiVersion: 1.20
Version: 1.8.3
GitCommit: f4bf5c7
Arch: amd64
GoVersion: go1.4.2
Step 0 : FROM ubuntu:14.04
 ---> c4bea91afef3
Step 1 : ARG 
Unknown instruction: ARG

I have found no way to pass a build-arg to the automated build, but my ARG's default value is sane, so that shouldn't matter.

Any ideas what the problem here is?

Answer 1

Docker Hub's automated build is still running Docker 1.8 and doesn't support ARG yet. There is a github issue tracking their upgrade to 1.9.

Answer 2

We encountered the same issue on Travis.ci. Upgrading to Docker Engine 1.9.0 worked for us.

We added this to our travis.yml

before_script:
  - sudo apt-get update
  - sudo apt-get install -y -q docker-engine=$DOCKER_ENGINE_VERSION