Jenkinsfile - Agents questions - docker

Question

I have two questions to following example:
pipeline {
agent { label "docker" }
stages {
stage('Build') {
agent {
docker {
image 'maven:3.5.0-jdk-8'
}
}
steps {
...
}
}
}
}
Question 1:
When I declare agent in top level of Jenkinsfile it means that it will be used for all below stages. So what is difference between:
agent { label "docker" }
and
agent {
docker {
image 'maven:3.5.0-jdk-8'
}
}
First one will use docker agent and second will use docker agent with maven image as executable environment? Where label "docker" agent is configured/installed?
Question 2:
How label tag is working? I know that somewhere is already created agent and using label I just point to it - like in example above: by default I use "docker" agent? it also means that during steps {...} this agent will be overridden by maven agent?
Question 3:
Last question for following example:
pipeline {
agent {
docker {
image 'maven:3-alpine'
args '-v ... -e ...'
}
}
stages {
stage('Maven Build') {
steps {
sh '...'
}
}
stage('Docker push') {
agent {
docker 'openjdk:8-jdk-alpine'
}
steps {
script {
docker.build("my-image")
}
}
}
}
post {
...
}
}
I want to build first stage using docker container with maven:3-alpine image. During build following error is printed:
...tmp/durable-54b54bdc/script.sh: line 1: docker: not found
So I modified this example, here is the working result:
pipeline {
agent any
stages {
stage('Docker push') {
steps {
script {
docker.build("my-image")
}
}
}
}
}
How is it working agent any in this case? Which agent can execute docker.build?

Answer 1

Answer 1:
agent { label "docker" }
This will try to find an agent with label docker and execute the steps in that agent.
agent {
docker {
image 'maven:3.5.0-jdk-8'
}
}
This will try to pull the docker image with name maven:3.x.x and start the container and execute the steps mentioned in the pipeline. If you are using MultiJob this will be executed in the slave with the label based on this configuration:
Answer 2:
Defining agent at the top-level of the Pipeline ensures that an Executor will be assigned to the agent labeled docker. To my knowledge, I assume that docker container will be created in the agent labeled docker and steps will be executed inside the container.
Answer 3:
The reason could be, you may not have configured Docker Label (refer above image). The task could have executed in the master where the docker is not installed. The reason for other one working could be because, the job is executed in an agent where docker is installed.