When running something like

node {
  docker.image('ubuntu').inside {
    def mvntool = tool name: "maven3.3.9", type: 'hudson.tasks.Maven$MavenInstallation'
    echo "Maven path: ${mvntool}"
  }
}

The Maven path in question is on the agent, not in the container. Which is reasonable, but inconvenient. Might we want to make sure the tools directory for the agent exists and is mapped into the container like the workspace is?