.gitignore an already committed folder

I accidentially committed a folder that is generated during each build automatically. It´s not necessary to have that folder under git control. To not have that folder tracked in the future anymore the following process can be applied:

  1. Commit the uncommitted changes in the repo
  2. Edit .gitignore to ignore the folder
  3. Issue the following three git commands
git rm -r --cached .  
git add .
git commit -m "Have working .gitignore"

The solution was described by Théo Attali on stackoverflow in 2017.The git-rm documentation explains the used options:

Allow recursive removal when a leading directory name is given.
-- cached
Use this option to unstage and remove paths only from the index. Working tree files, whether modified or not, will be left alone.
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