Add Portainer stack.env support for environment variable management

- Add docker/stack.env template for Portainer deployment
- Update Dockerfile to accept build args for Next.js environment variables
- Update docker-compose.deploy.yml to pass build args from environment
- Add comprehensive Portainer deployment guide in DOCKER_DEPLOYMENT.md
- Document environment variable setup for both CLI and Portainer deployments

Dockerfile

@@ -12,17 +12,23 @@ RUN pnpm install --frozen-lockfile --prod
 # Build stage
 FROM base AS builder
 RUN apk add --no-cache libc6-compat git postgresql-client
+
+# Accept build arguments for Next.js environment
+ARG NEXT_PUBLIC_VAPID_PUBLIC_KEY=dummy_key_for_build
+ARG BETTER_AUTH_SECRET=dummy_secret_for_build
+ARG BETTER_AUTH_URL=http://localhost:3000
+
+# Set environment variables for build
+ENV NEXT_PUBLIC_VAPID_PUBLIC_KEY=${NEXT_PUBLIC_VAPID_PUBLIC_KEY}
+ENV BETTER_AUTH_SECRET=${BETTER_AUTH_SECRET}
+ENV BETTER_AUTH_URL=${BETTER_AUTH_URL}
+
 COPY pnpm-lock.yaml pnpm-workspace.yaml package.json ./
 RUN pnpm install --frozen-lockfile
 
 # Copy source code
 COPY . .
 
-# Set required build-time environment variables
-ENV NEXT_PUBLIC_VAPID_PUBLIC_KEY=dummy_key_for_build
-ENV BETTER_AUTH_SECRET=dummy_secret_for_build
-ENV BETTER_AUTH_URL=http://localhost:3000
-
 # Build the app
 RUN pnpm build 2>&1