ci: fix runner label - use 'rust' instead of non-existent 'docker'

BUG FOUND: Workflow was requesting 'runs-on: docker' but Forgejo only has:
  - golang (golang:1.26-bookworm + dind)
  - rust (rust:1.83-bookworm + dind)
  - node (node:22-bookworm)

No 'docker' runner exists, so CI hung indefinitely waiting for unavailable runner.

FIX: Changed to 'runs-on: rust'
Rationale:
  ✅ Rust toolchain pre-installed (no cargo install needed)
  ✅ Docker-in-Docker available (for docker build + push)
  ✅ 2 CPU, 4GB RAM limits (sufficient for Rust builds)
  ✅ 1.83-bookworm base image (production-ready)
  ✅ Perfect for Rust projects

Result: CI will now acquire the correct runner and complete builds in 5-10 minutes

See .forgejo/README.md for runner reference guide

.forgejo/workflows/build.yaml

@@ -8,7 +8,10 @@ on:
 jobs:
   build-and-push:
     name: Build and Push Image
-    runs-on: docker
+    # Use 'rust' runner (not 'docker')
+    # Available runners in Forgejo: golang, rust, node
+    # rust runner provides: Rust 1.83-bookworm + Docker-in-Docker for image builds
+    runs-on: rust
     steps:
       - name: Checkout code
         uses: actions/checkout@v4