Pin the Docker image

Exercism's policy is to prefer pinned versions.
Using the same hash across all runners allows us to store and reuse the same image, rather than needing to store a per-runner base image. This helps cut down on storage costs.

Author: IsaacG

Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.18
+FROM alpine:3.18.12@sha256:de0eb0b3f2a47ba1eb89389859a9bd88b28e82f5826b6969ad604979713c2d4f
 
 # install packages required to run the tests
 RUN apk add --no-cache jq coreutils