Upgrade Dockerfile and CI from Node 18 to Node 24 (LTS)

Node 18 reached end-of-life on April 30, 2025. A transitive dependency
(undici@7.25.0) requires Node >=20.18.1, causing both the Docker build
and PR CI to fail during yarn install.

Upgrade to Node 24 (Active LTS since October 2025, EOL April 2028):
- Dockerfile: all three FROM stages updated to node:24-bookworm-slim
- pr.yaml: setup-node version updated to 24

Aligns with the Node 24 GitHub Actions runner upgrade in #52.

Author: salmanmkc

.github/workflows/pr.yaml

@@ -12,7 +12,7 @@ jobs:
         uses: actions/checkout@v4
       - uses: actions/setup-node@v4
         with:
-          node-version: 18
+          node-version: 24
       - name: Run tests
         run: |
           yarn install --frozen-lockfile --network-timeout 600000

Dockerfile

@@ -1,5 +1,5 @@
 # Stage 1 - Create yarn install skeleton layer
-FROM node:18-bookworm-slim AS packages
+FROM node:24-bookworm-slim AS packages
 
 WORKDIR /app
 COPY package.json yarn.lock ./
@@ -12,7 +12,7 @@ COPY plugins plugins
 RUN find packages \! -name "package.json" -mindepth 2 -maxdepth 2 -exec rm -rf {} \+
 
 # Stage 2 - Install dependencies and build packages
-FROM node:18-bookworm-slim AS build
+FROM node:24-bookworm-slim AS build
 
 USER node
 WORKDIR /app
@@ -34,7 +34,7 @@ RUN mkdir packages/backend/dist/skeleton packages/backend/dist/bundle \
     && tar xzf packages/backend/dist/bundle.tar.gz -C packages/backend/dist/bundle
 
 # Stage 3 - Build the actual backend image and install production dependencies
-FROM node:18-bookworm-slim
+FROM node:24-bookworm-slim
 
 # Install isolate-vm dependencies, these are needed by the @backstage/plugin-scaffolder-backend.
 # Install packages needed to get utility binaries