build: pin docker base images by digest

Floating tags node:22-alpine and nginx:1.27-alpine let upstream
swap image contents without review. Pin both FROM lines to their
current digest per the supply-chain policy, keeping the tag as
a trailing comment for readability.

Dockerfile

@@ -1,10 +1,12 @@
-FROM node:22-alpine AS build
+FROM node:22-alpine@sha256:16e22a550f3863206a3f701448c45f7912c6896a62de43add43bb9c86130c3e2 AS build
+# node:22-alpine
 WORKDIR /app
 COPY package.json package-lock.json ./
 RUN npm ci
 COPY . .
 RUN npm run build
 
-FROM nginx:1.27-alpine
+FROM nginx:1.27-alpine@sha256:65645c7bb6a0661892a8b03b89d0743208a18dd2f3f17a54ef4b76fb8e2f2a10
+# nginx:1.27-alpine
 COPY nginx.conf /etc/nginx/conf.d/default.conf
 COPY --from=build /app/dist /usr/share/nginx/html