fix(ci): stage native binary in build-output/ to bypass .dockerignore

The .dockerignore has 'target' which excludes the target/
directory from the docker build context. The Dockerfile was
COPYing the binary from target/, so docker build failed with
'file not found in build context or excluded by .dockerignore'.

Fix: copy the binary to build-output/ (a non-excluded path)
before docker build, and update the Dockerfile to copy from
build-output/.

- Add 'Stage binary for Docker' step that does:
    mkdir -p build-output
    cp target/shot-crafter-calculator-1.0.0-runner build-output/
- Dockerfile COPY now reads build-output/shot-crafter-calculator-1.0.0-runner
- build-output/ is not in .dockerignore -> only the binary
  (~117MB) ships in the build context, not the whole target/
  tree (~200MB with classes, generated-sources, node binaries, etc.)

.gitea/workflows/ci.yml

@@ -66,6 +66,11 @@ jobs:
         run: |
           ls -la target/shot-crafter-calculator-1.0.0-runner
 
+      - name: Stage binary for Docker
+        run: |
+          mkdir -p build-output
+          cp target/shot-crafter-calculator-1.0.0-runner build-output/
+
       - name: Build & Push Docker image
         if: github.event_name == 'push' && github.ref == 'refs/heads/main'
         run: |