pipeline does not use docker environment

Pipeline script removes the environment of the docker container for example the python example from the wiki does not work

pipeline {
  agent { docker 'python:2.7' }
  stages {
    stage('build') {
      steps {
        sh 'pip --version'
        sh 'python --version'
      }
    }
  }
}

It complains it can not find pip.
"-BLNTQZUKKFHPWKSPOCWYWUBGIXB3R5GHUR3CBX6DDZ4I7WKFS53Q@tmp/durable-61155246/script.sh: pip: not found"

Change to /usr/bin/pip and it works, not ideal.

As well as this '/usr/bin/pip install' does not work, as it can not write to system directories.
I know virtualenv could be used but this defeats the point of using docker, as docker is the temporary environment.