Update Gitpod Dockerfile

[mahesh-panchal]

• Tool installation changed to from mamba back to conda again to resolve Docker build error:

  ...
  319.1 Executing transaction: ...working... done
  342.8 No module named 'conda.cliconda'
  ...
  ERROR: failed to solve: process "/bin/sh -c conda config --add channels defaults &&     conda config --add channels bioconda &&     conda config --add channels conda-forge &&     conda config --set channel_priority strict &&     conda install --quiet --yes --name base mamba &&     mamba install --quiet --yes --name base         nextflow         nf-core         nf-test         black         prettier         pre-commit         pytest-workflow &&     mamba clean --all -f -y" did not complete successfully: exit code: 1
  

  Error is caused by mamba clean --all -f -y. Changing it to conda clean --all -f -y fixes the issue.
  Since conda is now using libmamba as a solver, updated to just use conda to install it all.

• Added comment how to test image locally.

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[maxulysse]

sounds not only sensible, but the right thing to do