fix(security): harden Dockerfiles - secure defaults, signal handling, healthcheck

[lakhansamani]

Summary

• Signal handling (HIGH): Added exec before ./authorizer in all deployment Dockerfiles so the binary receives SIGTERM directly instead of being SIGKILLed after timeout
• Public demo defaults (CRITICAL): Changed ENABLE_PLAYGROUND, ENABLE_GRAPHQL_INTROSPECTION to false and DISABLE_ADMIN_HEADER_AUTH to false by default
• HEALTHCHECK (MEDIUM): Added HEALTHCHECK directive to main Dockerfile for container orchestrator health monitoring

Files Changed

• Dockerfile — HEALTHCHECK added
• Deployment Dockerfiles updated in their respective repos

Test plan

• Verify docker build succeeds
• Verify container responds to SIGTERM gracefully
• Verify healthcheck reports healthy when app is running