fix(Dockerfile): setup gnupg correctly for nobody

[gagantrivedi]

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• I have added information to docs/ if required so people know about the feature!
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• I have filled in the "How did you test this code" section below?
• I have used a Conventional Commit title for this Pull Request

Changes

• Make nobody owner of .gnupg
• pass gnupghome to GPG

How did you test this code?

By deploying in staging and verifying that the data in influx is available

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Name	Status	Preview	Updated (UTC)
docs	✅ Ready (Inspect)	Visit Preview	Dec 14, 2023 1:30pm
flagsmith-frontend-preview	✅ Ready (Inspect)	Visit Preview	Dec 14, 2023 1:30pm
flagsmith-frontend-staging	✅ Ready (Inspect)	Visit Preview	Dec 14, 2023 1:30pm

[github-actions]

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9c475e4) 96.11% compared to head (ca6146e) 96.08%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3167      +/-   ##
==========================================
- Coverage   96.11%   96.08%   -0.04%     
==========================================
  Files        1058     1059       +1     
  Lines       31918    32534     +616     
==========================================
+ Hits        30677    31259     +582     
- Misses       1241     1275      +34     

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