Tasks fail to run without even starting

I have a simple pipeline defined as follows:

clone source code from GitLab private repo
run two parallel tasks:
- postgres as long lived service
- Ruby on Rails tests via RSpec
build docker image with in-source Dockerfile

Source code cloning works fine, but then the test task fails with the following error.

Same with a simpler pipeline that just builds a docker image of a Node application (no tests, no db) with a Dockerfile that I wrote through the pipeline UI.

What am I missing? Why is it looking for a Backend env-var?
For the record, the Ruby pipeline is called ProjectName Backend, while the NodeJS one is called ProjectName Prod. In the latter case, the missing envar is Prod.

Rancher version: 1.6.13
Docker version: 17.09.1-ce
Single node deploy (6 cores, 24 GB of RAM, CentOS 7)
Pipelines installed through Rancher catalog.