fix: bind docs server and preserve runtime port defaults

[9trocode]

Root cause

Docusaurus defaults to localhost when --host is omitted, so Docker port publishing cannot reach the server. A subsequent direct main change added configurable HOST and PORT build args before FROM, but Docker does not expose those values inside the production stage unless they are redeclared. That produced commands such as docusaurus serve --host --port 8000 and the container exited.

Fix

• declare ENV, PORT, and HOST build arguments inside the production stage
• default to HOST=0.0.0.0 and PORT=8000
• preserve PipeOps runtime overrides such as PORT=3000
• quote both runtime values in the shell command

Verification

Reproduced before the fix:

• original image logged http://localhost:8000/; Docker-published traffic failed
• current main image exited with docusaurus serve --host --port 8000 when only PORT was provided

Verified after the fix:

• Docker build succeeds without undefined-variable warnings
• default runtime serves http://0.0.0.0:8000/ and returns HTTP 200
• PORT=3000 serves http://0.0.0.0:3000/ and returns HTTP 200
• Docusaurus production build completes successfully