fix(docker): add HTTP_PROXY/HTTPS_PROXY build args for selfhost

[guunergooner]

Summary

• Add HTTP_PROXY, HTTPS_PROXY, NO_PROXY build arguments to Dockerfile, Dockerfile.web, and docker-compose.selfhost.build.yml
• Enables selfhost deployments behind corporate proxies to pass proxy settings at build time

Test plan

• Build with docker compose -f docker-compose.selfhost.build.yml build without proxy env vars (default behavior unchanged)
• Build with HTTP_PROXY/HTTPS_PROXY set and verify args are passed into build stages

[vercel]

@guunergooner is attempting to deploy a commit to the IndexLabs Team on Vercel.

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