Pin the Docker image (#184)

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 +1,14 @@
-FROM alpine:3.18
+# Prefer a pinned hash that matches what is used in other test runners.
+#
+# alpine:3.18.12@sha256:de0eb0b3f2a47ba1eb89389859a9bd88b28e82f5826b6969ad604979713c2d4f
+# alpine:3.20.10@sha256:d9e853e87e55526f6b2917df91a2115c36dd7c696a35be12163d44e6e2a4b6bc
+# alpine:3.22.4@sha256:310c62b5e7ca5b08167e4384c68db0fd2905dd9c7493756d356e893909057601
+# alpine:3.23.4@sha256:5b10f432ef3da1b8d4c7eb6c487f2f5a8f096bc91145e68878dd4a5019afde11
+# debian:bookworm-slim@sha256:f9c6a2fd2ddbc23e336b6257a5245e31f996953ef06cd13a59fa0a1df2d5c252
+# ubuntu:22.04@sha256:962f6cadeae0ea6284001009daa4cc9a8c37e75d1f5191cf0eb83fe565b63dd7
+# ubuntu:24.04@sha256:c4a8d5503dfb2a3eb8ab5f807da5bc69a85730fb49b5cfca2330194ebcc41c7b
+# ubuntu:26.04@sha256:f3d28607ddd78734bb7f71f117f3c6706c666b8b76cbff7c9ff6e5718d46ff64
+FROM alpine:3.18.12@sha256:de0eb0b3f2a47ba1eb89389859a9bd88b28e82f5826b6969ad604979713c2d4f
 
 # install packages required to run the tests
 RUN apk add --no-cache jq coreutils