fix(Dockerfile): Use correct secret ID for pgp_key

[gagantrivedi]

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Changes

Use correct secret ID in Dockerfile

How did you test this code?

By running the image build here: https://github.com/Flagsmith/flagsmith/actions/runs/7176928514 locally to make sure gpg is installed

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Name	Status	Preview	Updated (UTC)
docs	✅ Ready (Inspect)	Visit Preview	Dec 12, 2023 6:00am
flagsmith-frontend-preview	✅ Ready (Inspect)	Visit Preview	Dec 12, 2023 6:00am
flagsmith-frontend-staging	✅ Ready (Inspect)	Visit Preview	Dec 12, 2023 6:00am

[github-actions]

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (068253e) 96.08% compared to head (2c8b8b1) 96.08%.
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3141      +/-   ##
==========================================
- Coverage   96.08%   96.08%   -0.01%     
==========================================
  Files        1054     1054              
  Lines       31702    31699       -3     
==========================================
- Hits        30462    30459       -3     
  Misses       1240     1240              

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