fix: always redownload the NGINX GPG signing key in the NGINX OSS Dockerfile

[alessfg]

Proposed changes

This PR does two things:

• Modifies the S3 Gateway NGINX OSS Dockerfile to redownload the NGINX GPG signing key in order to avoid issues with expired signing keys.
• Refactors the Dockerfile into a more sensible structure. Splits the RUN block into a block dedicated to installing NJS (which should remain relatively static) and a block dedicated to update the actual NJS S3 Gateway codebase.

Checklist

Before creating a PR, run through this checklist and mark each as complete:

• I have read the contributing guidelines.
• I have signed the F5 Contributor License Agreement (CLA).
• If applicable, I have added tests that prove my fix is effective or that my feature works.
• If applicable, I have checked that any relevant tests pass after adding my changes.
• I have updated any relevant documentation (e.g. README.md).

[4141done]

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