I'm noticing  different behavior after upgrading from Jenkins 1.x to 2.121.2 in how PATH is set.   The withEnv pipeline step includes a special syntax of PATH+ which will update the PATH.  However, this is no longer setting the PATH when nested under the Docker Pipeline docker.image

In the example below, $MYAPP is available on the output under the PATH var in both nodes below in version 1.642.4.  However, $MYAPP is not set in the PATH output on the node using the docker.image step on Jenkins 2.121.2

//ADDS $MYAPP to $PATH
node {
 withEnv(["PATH+MYAPP=/path/to/myapp/bin"]) {
   stage('Test') {
     sh 'printenv | sort'
   }
  }
}
//DOESN'T ADD $MYAPP to $PATH
node {
 docker.image('bash:latest').inside {
   withEnv(["PATH+MYAPP=/path/to/myapp/bin"]) {
     stage('Test') {
       sh 'printenv | sort'
     }
    }
  }
}