fix(docker): build on Node 20 (fast-jwt requires node >=20)

The final image stage failed `yarn install --production` because the
transitive dependency fast-jwt now requires Node >=20 while the image was
built on node:18-slim (also EOL). Bump both base stages to node:20-slim.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: n4ze3m

Dockerfile

@@ -1,4 +1,4 @@
-FROM node:18-slim AS base
+FROM node:20-slim AS base
 WORKDIR /app
 RUN apt update && apt install -y \
     g++ make python3 wget gnupg dirmngr unzip
@@ -18,7 +18,7 @@ COPY . .
 RUN pnpm install && pnpm build
 
 # Final stage
-FROM node:18-slim
+FROM node:20-slim
 WORKDIR /app
 
 # Set environment variables