fix: use public.ecr.aws/docker/library/alpine

We're getting rate limited by docker.io when pulling the alpine docker
image.

We're changing it to a different registry that's less prone to rate
limiting.

Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.15.0
+FROM public.ecr.aws/docker/library/alpine:3.15.0
 
 RUN apk update && apk --no-cache add curl jq coreutils