chore: pin Docker image by digest

Author: nosborn

Dockerfile

@@ -1,6 +1,6 @@
-FROM node:lts-alpine
+FROM node:lts-alpine@sha256:41e4389f3d988d2ed55392df4db1420ad048ae53324a8e2b7c6d19508288107e
 
-RUN npm install --global --production --update-notifier=false markdownlint-cli@0.43.0
+RUN npm install --global --omit=dev --update-notifier=false markdownlint-cli@0.43.0
 
 COPY entrypoint.sh /entrypoint.sh
 COPY markdownlint-problem-matcher.json /markdownlint-problem-matcher.json