fix(docker): add apt-get upgrade to address security vulnerabilities

[usmansaleem]

PR Description

Include apt-get upgrade -y in the base image setup to ensure all
packages receive security updates. This addresses CVE-2025-68973
in gpgv and other potential vulnerabilities in the base image.

Fixed Issue(s)

Documentation

• I thought about documentation and added the doc-change-required label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.

Testing

• I thought about testing these changes in a realistic/non-local environment.

---

Note

Improves container security by ensuring base image packages are fully patched during build and documenting the change.

• docker/Dockerfile: adds apt-get upgrade -y to apply all available security updates during build; retains minimal deps and non-root web3signer user
• CHANGELOG.md: new Security notes highlighting mitigation for CVE-2025-68973 in gpgv and that images now apply all security updates during build

^{Written by Cursor Bugbot for commit a6cd012. This will update automatically on new commits. Configure here.}

[joshuafernandes]

LGTM