feat(docker): add containerized nginx deployment with runtime base path

Two-stage build: node builds the static bundle, nginx serves it. The bundle is
built against a placeholder base token that the entrypoint rewrites to $BASE_URL
at start, so one image serves from any path without a rebuild.

nginx config caches hashed assets forever, never caches index.html, and applies
security headers (CSP, no framing, no referrer) suited to an app that handles
cloud credentials client-side.

.gitattributes pins LF on the container-consumed files, since core.autocrlf
would otherwise give the entrypoint a CRLF shebang.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitattributes

@@ -0,0 +1,6 @@
+# core.autocrlf is on for this repo; these files are consumed by Linux inside the
+# container, where a CRLF shebang fails as "no such file or directory".
+*.sh        text eol=lf
+*.conf      text eol=lf
+*.template  text eol=lf
+Dockerfile  text eol=lf