[#4874] Add missing script to the Dockerfile

[vaszig]

Closes #4874

Changes

• Added fix_selectboxes_component_default_values scipt to the Dockerfile
• Updated issue template (reminder for such scripts)
• Added instructions for downloading and adding the script manually

Checklist

Check off the items that are completed or not relevant.

• Impact on features

  • Checked copying a form
  • Checked import/export of a form
  • Config checks in the configuration overview admin page
  • Problem detection in the admin email digest is handled

• Release management

  • I have labelled the PR as "needs-backport" accordingly

• I have updated the translations assets (you do NOT need to provide translations)

  • Ran ./bin/makemessages_js.sh
  • Ran ./bin/compilemessages_js.sh

• Commit hygiene

  • Commit messages refer to the relevant Github issue
  • Commit messages explain the "why" of change, not the how

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.57%. Comparing base (e776c7b) to head (dcce330).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4898   +/-   ##
=======================================
  Coverage   96.57%   96.57%           
=======================================
  Files         757      757           
  Lines       25728    25728           
  Branches     3379     3379           
=======================================
  Hits        24847    24847           
  Misses        616      616           
  Partials      265      265           

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[sergei-maertens]

Backports:

• 2.8.x: 8719238
• 2.7.x: b04dd43