fix: add build-time environment variables for Docker build

[productdevbook]

Summary

• Fixed Docker build failure caused by missing environment variables during build process
• Added ARG declarations and ENV setup in Dockerfile build stage
• Added build-args to GitHub Actions workflow with dummy values for build-time usage

Problem

The GitHub Actions Docker build was failing with exit code 1 because the Nuxt build process requires environment variables (DATABASE_URL, JWT_SECRET, ENCRYPTION_KEY, WEBHOOK_SECRET) during build time for GraphQL schema generation and other build processes.

Solution

• Added ARG declarations in Dockerfile for all required environment variables
• Set ENV variables in build stage using ARG values
• Added build-args to GitHub Actions workflow with dummy values (safe for build-time usage)
• These dummy values are only used during build and don't affect runtime security

Test plan

• Docker build now includes required environment variables during build stage
• GitHub Actions workflow provides build arguments for successful builds
• Verify GitHub Actions workflow runs successfully with these changes
• Confirm Docker images build and publish correctly

Fixes the build failure referenced in GitHub Actions workflow run.