Add Docker setup and fix proxy config for containerized deployment

- Program.cs: remove UseHsts/UseHttpsRedirection (NPM terminates TLS,
  would loop behind the proxy, plan.md AE-4), add UseForwardedHeaders
  with emptied known networks/proxies. Uses KnownIPNetworks (net10).
- Dockerfile: SDK build stage, chiseled aspnet runtime on port 8080.
- .dockerignore: keep build output, git and docs out of the image.
- compose.yaml: temporary test setup (builds from source, exposes
  5000:8080) for the first run on Unraid without NPM or registry.
- .gitignore: exclude .idea/ (Rider project files).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.dockerignore

@@ -0,0 +1,22 @@
+# Build-Ausgaben (werden im Container frisch erzeugt)
+**/bin/
+**/obj/
+**/out/
+
+# IDE- und Tooling-Kram
+.vs/
+.idea/
+.vscode/
+
+# Versionskontrolle und Doku, im Image nicht benoetigt
+.git/
+.gitea/
+.gitignore
+.gitattributes
+docs/
+*.md
+
+# Docker-Dateien selbst
+Dockerfile
+.dockerignore
+compose.yaml