fix(examples): pull gin base images from ECR Public mirror (#752)

The gin example was the only one still pulling its base images
(golang:1.18-alpine, alpine:3.9) directly from Docker Hub, which
caused the test-image (gin) CI job to fail with registry-1.docker.io
connection timeouts (Docker Hub anonymous pull rate limits).

Switch both stages to the public.ecr.aws/docker/library mirror to
match every other example, and bump to golang:1.23-alpine / alpine:3.20.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
Co-authored-by: Harold Sun <sunhua@amazon.com>

Author: 3 people

examples/gin/app/Dockerfile

@@ -1,12 +1,12 @@
-FROM golang:1.18-alpine AS build_base
+FROM public.ecr.aws/docker/library/golang:1.23-alpine AS build_base
 RUN apk add --no-cache git
 WORKDIR /tmp/gin
 
 COPY . .
 RUN go mod download
 
 RUN GOOS=linux CGO_ENABLED=0 go build -o bootstrap .
-FROM alpine:3.9 
+FROM public.ecr.aws/docker/library/alpine:3.20
 RUN apk add ca-certificates
 COPY --from=public.ecr.aws/awsguru/aws-lambda-adapter:1.0.1 /lambda-adapter /opt/extensions/lambda-adapter
 COPY --from=build_base /tmp/gin/bootstrap /app/bootstrap