Jenkins docker container simply hangs and never executes steps - docker

Question

I'm trying to run a Python image in Jenkins to perform a series of unit tests with pytest, but I'm getting some strange behavior with Docker.
My Jenkinsfile pipeline is
agent {
docker { image 'python:3.6-jessie' }
}
stages {
stage('Run tests') {
steps {
withCredentials([
string(credentialsId: 'a-secret', variable: 'A_SECRET')
{
sh label: "Install dependencies", script: 'pip install -r requirements.txt'
sh label: 'Execute tests', script: "pytest mytests.py"
}
}
}
}
However, when I run the pipeline, Docker appears to be executing a very long instruction (with significantly more -e environment variables than I defined as credentials?), followed by cat.
The build then simply hangs and never finishes:
Jenkins does not seem to be running inside a container
$ docker run -t -d -u 996:994
-w /var/lib/jenkins/workspace/myproject
-v /var/lib/jenkins/workspace/myproject:/var/lib/jenkins/workspace/myproject:rw,z
-v /var/lib/jenkins/workspace/myproject#tmp:/var/lib/jenkins/workspace/myproject#tmp:rw,z
-e ******** -e ******** python:3.6-jessie cat
When I SSH into my instance and run docker ps, I see
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
240d00459d92 python:3.6-jessie "cat" About a minute ago Up About a minute kind_wright
Why is Jenkins running cat? Why does Jenkins say I am not running inside a container, when it has clearly created a container for me? And most importantly, why are my pip install -r requirements and other steps not executing?

Answer 1

I finally figured this out. If you have empty global environment variables in your Jenkins configuration, it appears that you'll get a malformed docker run command since Jenkins will write the command, with your empty string environment variable, as docker run -e some_env_var=some_value -e = ...
This will cause the container to simply hang.
A telltale sign that this is happening is you'll get the error message:
invalid argument "=" for "-e, --env" flag: invalid environment variable: =
This is initially difficult to diagnose since Jenkins (rightfully) hides your actual credentials with ***, so the empty environment strings do not show up as empty.
You need to check your Jenkins global configuration and make sure you don't have any empty environment variables accidentally defined:
If these exist, you need to delete them and rerun.