fix(examples): pull gin base images from ECR Public mirror

[JamBalaya56562]

Problem

The test-image (gin) CI job (Verify Examples) was failing at the Build step (sam build) with Docker Hub connection timeouts:

Step 1/13 : FROM golang:1.18-alpine AS build_base
Get "https://registry-1.docker.io/v2/": net/http: request canceled (Client.Timeout exceeded)
...
Get "https://registry-1.docker.io/v2/": context deadline exceeded

All 5 backoff retries were exhausted by unreachable registry-1.docker.io, so the job exited 1.

Root cause

examples/gin/app/Dockerfile was the only example still pulling its base images directly from Docker Hub (golang:1.18-alpine, alpine:3.9). Every other example already uses the ECR Public mirror (public.ecr.aws/docker/library/...). Anonymous Docker Hub pulls from GitHub Actions runners are rate-limited / flaky, which caused the failure.

The test-zip (gin-zip) job passed because the zip variant does not build the Docker image and thus never pulls from Docker Hub — consistent with the failure being Docker Hub-specific.

Change

examples/gin/app/Dockerfile:

• golang:1.18-alpine → public.ecr.aws/docker/library/golang:1.23-alpine
• alpine:3.9 → public.ecr.aws/docker/library/alpine:3.20

This aligns gin with all other examples and bumps the stale base image versions. go.mod declares go 1.18, so golang 1.23 builds it with backward compatibility.

Verification

• Local docker build of examples/gin/app succeeds, pulling base images via public.ecr.aws (no Docker Hub access).
• Ran the Verify Examples workflow on this branch via workflow_dispatch: all jobs green, including test-image (gin) passing in ~1m8s (previously failed after 5min of retries).

🤖 Generated with Claude Code