docs: run bash explicitly for migration

[nicosqc]

hope this is sufficient, else let me know if you would change something. Bash won't be a login shell but since we clear the enviroment with sudo -i this should not have any meaningful impact

[Spitfireap]

Thanks, it will help, surely. There are a few others sudo -i across the documentation, would you mind editing all of them ? :)

[nicosqc]

struggled with my local branch for a bit there (and github closed it therefore) but updated the fork to change all occurrences

[nicosqc]

something went wrong idk. why does it say i force pushed to 572e32f when i added https://github.com/nicosqc/modoboa/commit/21bf16317c4acece6c498365d1229e4dab65031f

€: oh well, guess it's fine cause changed files and commits have my edits, seems like thats just an old state from me messing with my branch

[codecov]

Codecov Report

Merging #2717 (21bf163) into master (91e779b) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2717   +/-   ##
=======================================
  Coverage   82.47%   82.47%           
=======================================
  Files         211      211           
  Lines       11089    11089           
  Branches     1727     1727           
=======================================
  Hits         9146     9146           
  Misses       1459     1459           
  Partials      484      484