Invalid reference format error when auto-building repository

Hi there,

I’ve set up docker builds for a github repository.

For my builds I’m using a publicly available base image.

A specific tag of the base image is used by my builder.

For example:

FROM bryanhuntesl/artful-erlang:$otp_tag

Where otp_tag is :


This was all working last week, but is now breaking with an ‘invalid reference format’ error :

Building in Docker Cloud's infrastructure...
Cloning into '.'...
Warning: Permanently added the RSA host key for IP address '' to the list of known hosts.
Reset branch 'master'
Your branch is up-to-date with 'origin/master'.
KernelVersion: 4.4.0-93-generic
Arch: amd64
BuildTime: 2017-08-17T22:50:04.828747906+00:00
ApiVersion: 1.30
Version: 17.06.1-ce
MinAPIVersion: 1.12
GitCommit: 874a737
Os: linux
GoVersion: go1.8.3
Starting build of
Step 1/35 : ARG otp_tag
Step 2/35 : FROM bryanhuntesl/artful-erlang:$otp_tag AS artful_riak_builder
invalid reference format
ERROR: Build failed: invalid reference format
ERROR: Build failed with exit code 2
ERROR: Build in 'master:/builders/context:/dockerfiles/artful-riak' (3fa026ea) failed in 0:00:15

Screenshot of how it’s configured :

Anyone got any suggestions?



Meaningless error message - found out more about the way it works - here -

Just a thought: Have you tried also the ${otp_tag} syntax?