[docs] Update ReFrame tutorial containers

[vkarak]

This is a follow up of #3651.

This PR also provides the following updates:

1. The Docker compose ReFrame tutorial container does not clone ReFrame from Github but uses the already cloned copy; this is useful also for development.
2. The reference to the requirements.txt is removed from the docs.

Closes #3654.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.87%. Comparing base (deb7f90) to head (5dd9cf0).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3661      +/-   ##
===========================================
- Coverage    91.87%   91.87%   -0.01%     
===========================================
  Files           62       62              
  Lines        13755    13755              
===========================================
- Hits         12638    12637       -1     
- Misses        1117     1118       +1     

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[ekouts]

I have a very minor comment. Other than this, it looks good.