Consider pinning Docker base images to SHA256 for reproducibility

[adnaan]

Context

This follow-up task was identified during the review of PR #132.

Source PR: #132
PR Title: feat: add Docker support with multi-stage build
Suggested by: @claude[bot]

Task Description

For enhanced reproducibility and security, consider pinning base images to specific SHA256 digests rather than tags.

Example:

FROM node:20-alpine@sha256:abc123... AS client-builder
FROM golang:1.25-alpine@sha256:def456... AS go-builder
FROM alpine:3.21@sha256:ghi789...

Trade-offs

Pros:

• Guaranteed reproducible builds
• Protection against supply chain attacks
• Immutable base images

Cons:

• Manual updates required for security patches
• More maintenance overhead

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This issue was automatically created from PR review comments.