Bitbucket pipelines not showing content of environment variables

image: python:3.7.3 

pipelines:

  branches:
    Server:
    - step: 
       name: Test
       script:
          - echo $MYTEST

My pipeline shows this:

echo $MYTEST

+ echo $MYTEST
$MYTEST

The variable is a "Repository variable" and is named MYTEST

I have done the same process with a node image and it is working fine. Because this is a python project i took python, but the environment variables don't seem to work.

I suppose, i cannot use a pure python docker image, if i need some bash commands, correct?